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331 lines
18 KiB
Plaintext
331 lines
18 KiB
Plaintext
2018-02-21 - Layering in haproxy 1.9
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------------------------------------
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2 main zones :
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- application : reads from conn_streams, writes to conn_streams, often uses
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streams
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- connection : receives data from the network, presented into buffers
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available via conn_streams, sends data to the network
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The connection zone contains multiple layers which behave independently in each
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direction. The Rx direction is activated upon callbacks from the lower layers.
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The Tx direction is activated recursively from the upper layers. Between every
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two layers there may be a buffer, in each direction. When a buffer is full
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either in Tx or Rx direction, this direction is paused from the network layer
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and the location where the congestion is encountered. Upon end of congestion
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(cs_recv() from the upper layer, of sendto() at the lower layers), a
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tasklet_wakeup() is performed on the blocked layer so that suspended operations
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can be resumed. In this case, the Rx side restarts propagating data upwards
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from the lowest blocked level, while the Tx side restarts propagating data
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downwards from the highest blocked level. Proceeding like this ensures that
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information known to the producer may always be used to tailor the buffer sizes
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or decide of a strategy to best aggregate data. Additionally, each time a layer
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is crossed without transformation, it becomes possible to send without copying.
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The Rx side notifies the application of data readiness using a wakeup or a
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callback. The Tx side notifies the application of room availability once data
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have been moved resulting in the uppermost buffer having some free space.
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When crossing a mux downwards, it is possible that the sender is not allowed to
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access the buffer because it is not yet its turn. It is not a problem, the data
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remains in the conn_stream's buffer (or the stream one) and will be restarted
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once the mux is ready to consume these data.
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cs_recv() -------. cs_send()
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^ +--------> |||||| -------------+ ^
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| | -------' | | stream
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--|----------|-------------------------------|-------|-------------------
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| | V | connection
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data .---. | | room
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ready! |---| |---| available!
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|---| |---|
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|---| |---|
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| | '---'
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^ +------------+-------+ |
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| | ^ | /
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/ V | V /
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/ recvfrom() | sendto() |
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-------------|----------------|--------------|---------------------------
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| | poll! V kernel
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The cs_recv() function should act on pointers to buffer pointers, so that the
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callee may decide to pass its own buffer directly by simply swapping pointers.
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Similarly for cs_send() it is desirable to let the callee steal the buffer by
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swapping the pointers. This way it remains possible to implement zero-copy
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forwarding.
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Some operation flags will be needed on cs_recv() :
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- RECV_ZERO_COPY : refuse to merge new data into the current buffer if it
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will result in a data copy (ie the buffer is not empty), unless no more
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than XXX bytes have to be copied (eg: copying 2 cache lines may be cheaper
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than waiting and playing with pointers)
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- RECV_AT_ONCE : only perform the operation if it will result in the source
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buffer to become empty at the end of the operation so that no two buffers
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remain allocated at the end. It will most of the time result in either a
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small read or a zero-copy operation.
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- RECV_PEEK : retrieve a copy of pending data without removing these data
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from the source buffer. Maybe an alternate solution could consist in
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finding the pointer to the source buffer and accessing these data directly,
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except that it might be less interesting for the long term, thread-wise.
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- RECV_MIN : receive minimum X bytes (or less with a shutdown), or fail.
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This should help various protocol parsers which need to receive a complete
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frame before proceeding.
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- RECV_ENOUGH : no more data expected after this read if it's of the
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requested size, thus no need to re-enable receiving on the lower layers.
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- RECV_ONE_SHOT : perform a single read without re-enabling reading on the
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lower layers, like we currently do when receiving an HTTP/1 request. Like
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RECV_ENOUGH where any size is enough. Probably that the two could be merged
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(eg: by having a MIN argument like RECV_MIN).
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Some operation flags will be needed on cs_send() :
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- SEND_ZERO_COPY : refuse to merge the presented data with existing data and
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prefer to wait for current data to leave and try again, unless the consumer
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considers the amount of data acceptable for a copy.
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- SEND_AT_ONCE : only perform the operation if it will result in the source
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buffer to become empty at the end of the operation so that no two buffers
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remain allocated at the end. It will most of the time result in either a
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small write or a zero-copy operation.
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Both operations should return a composite status :
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- number of bytes transferred
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- status flags (shutr, shutw, reset, empty, full, ...)
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2018-07-23 - Update after merging rxbuf
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---------------------------------------
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It becomes visible that the mux will not always be welcome to decode incoming
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data because it will sometimes imply extra memory copies and/or usage for no
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benefit.
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Ideally, when when a stream is instantiated based on incoming data, these
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incoming data should be passed and the upper layers called, but it should then
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be up these upper layers to peek more data in certain circumstances. Typically
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if the pending connection data are larger than what is expected to be passed
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above, it means some data may cause head-of-line blocking (HOL) to other
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streams, and needs to be pushed up through the layers to let other streams
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continue to work. Similarly very large H2 data frames after header frames
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should probably not be passed as they may require copies that could be avoided
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if passed later. However if the decoded frame fits into the conn_stream's
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buffer, there is an opportunity to use a single buffer for the conn_stream
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and the channel. The H2 demux could set a blocking flag indicating it's waiting
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for the upper stream to take over demuxing. This flag would be purged once the
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upper stream would start reading, or when extra data come and change the
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conditions.
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Forcing structured headers and raw data to coexist within a single buffer is
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quite challenging for many code parts. For example it's perfectly possible to
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see a fragmented buffer containing series of headers, then a small data chunk
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that was received at the same time, then a few other headers added by request
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processing, then another data block received afterwards, then possibly yet
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another header added by option http-send-name-header, and yet another data
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block. This causes some pain for compression which still needs to know where
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compressed and uncompressed data start/stop. It also makes it very difficult
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to account the exact bytes to pass through the various layers.
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One solution consists in thinking about buffers using 3 representations :
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- a structured message, which is used for the internal HTTP representation.
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This message may only be atomically processed. It has no clear byte count,
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it's a message.
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- a raw stream, consisting in sequences of bytes. That's typically what
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happens in data sequences or in tunnel.
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- a pipe, which contains data to be forwarded, and that haproxy cannot have
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access to.
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The processing efficiency decreases with the higher complexity above, but the
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capabilities increase. The structured message can contain anything including
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serialized data blocks to be processed or forwarded. The raw stream contains
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data blocks to be processed or forwarded. The pipe only contains data blocks
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to be forwarded. The the latter ones are only an optimization of the former
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ones.
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Thus ideally a channel should have access to all such 3 storage areas at once,
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depending on the use case :
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(1) a structured message,
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(2) a raw stream,
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(3) a pipe
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Right now a channel only has (2) and (3) but after the native HTTP rework, it
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will only have (1) and (3). Placing a raw stream exclusively in (1) comes with
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some performance drawbacks which are not easily recovered, and with some quite
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difficult management still involving the reserve to ensure that a data block
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doesn't prevent headers from being appended. But during header processing, the
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payload may be necessary so we cannot decide to drop this option.
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A long-term approach would consist in ensuring that a single channel may have
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access to all 3 representations at once, and to enumerate priority rules to
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define how they interact together. That's exactly what is currently being done
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with the pipe and the raw buffer right now. Doing so would also save the need
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for storing payload in the structured message and void the requirement for the
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reserve. But it would cost more memory to process POST data and server
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responses. Thus an intermediary step consists in keeping this model in mind but
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not implementing everything yet.
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Short term proposal : a channel has access to a buffer and a pipe. A non-empty
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buffer is either in structured message format OR raw stream format. Only the
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channel knows. However a structured buffer MAY contain raw data in a properly
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formatted way (using the envelope defined by the structured message format).
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By default, when a demux writes to a CS rxbuf, it will try to use the lowest
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possible level for what is being done (i.e. splice if possible, otherwise raw
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stream, otherwise structured message). If the buffer already contains a
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structured message, then this format is exclusive. From this point the MUX has
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two options : either encode the incoming data to match the structured message
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format, or refrain from receiving into the CS's rxbuf and wait until the upper
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layer request those data.
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This opens a simplified option which could be suited even for the long term :
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- cs_recv() will take one or two flags to indicate if a buffer already
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contains a structured message or not ; the upper layer knows it.
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- cs_recv() will take two flags to indicate what the upper layer is willing
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to take :
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- structured message only
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- raw stream only
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- any of them
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From this point the mux can decide to either pass anything or refrain from
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doing so.
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- the demux stores the knowledge it has from the contents into some CS flags
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to indicate whether or not some structured message are still available, and
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whether or not some raw data are still available. Thus the caller knows
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whether or not extra data are available.
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- when the demux works on its own, it refrains from passing structured data
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to a non-empty buffer, unless these data are causing trouble to other
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streams (HOL).
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- when a demux has to encapsulate raw data into a structured message, it will
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always have to respect a configured reserve so that extra header processing
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can be done on the structured message inside the buffer, regardless of the
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supposed available room. In addition, the upper layer may indicate using an
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extra recv() flag whether it wants the demux to defragment serialized data
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(for example by moving trailing headers apart) or if it's not necessary.
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This flag will be set by the stream interface if compression is required or
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if the http-buffer-request option is set for example. Probably that using
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to_forward==0 is a stronger indication that the reserve must be respected.
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- cs_recv() and cs_send() when fed with a message, should not return byte
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counts but message counts (i.e. 0 or 1). This implies that a single call to
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either of these functions cannot mix raw data and structured messages at
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the same time.
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At this point it looks like the conn_stream will have some encapsulation work
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to do for the payload if it needs to be encapsulated into a message. This
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further magnifies the importance of *not* decoding DATA frames into the CS's
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rxbuf until really needed.
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The CS will probably need to hold indication of what is available at the mux
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level, not only in the CS. Eg: we know that payload is still available.
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Using these elements, it should be possible to ensure that full header frames
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may be received without enforcing any reserve, that too large frames that do
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not fit will be detected because they return 0 message and indicate that such
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a message is still pending, and that data availability is correctly detected
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(later we may expect that the stream-interface allocates a larger or second
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buffer to place the payload).
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Regarding the ability for the channel to forward data, it looks like having a
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new function "cs_xfer(src_cs, dst_cs, count)" could be very productive in
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optimizing the forwarding to make use of splicing when available. It is not yet
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totally clear whether it will split into "cs_xfer_in(src_cs, pipe, count)"
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followed by "cs_xfer_out(dst_cs, pipe, count)" or anything different, and it
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still needs to be studied. The general idea seems to be that the receiver might
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have to call the sender directly once they agree on how to transfer data (pipe
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or buffer). If the transfer is incomplete, the cs_xfer() return value and/or
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flags will indicate the current situation (src empty, dst full, etc) so that
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the caller may register for notifications on the appropriate event and wait to
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be called again to continue.
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Short term implementation :
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1) add new CS flags to qualify what the buffer contains and what we expect
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to read into it;
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2) set these flags to pretend we have a structured message when receiving
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headers (after all, H1 is an atomic header as well) and see what it
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implies for the code; for H1 it's unclear whether it makes sense to try
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to set it without the H1 mux.
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3) use these flags to refrain from sending DATA frames after HEADERS frames
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in H2.
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4) flush the flags at the stream interface layer when performing a cs_send().
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5) use the flags to enforce receipt of data only when necessary
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We should be able to end up with sequential receipt in H2 modelling what is
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needed for other protocols without interfering with the native H1 devs.
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2018-08-17 - Considerations after killing cs_recv()
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---------------------------------------------------
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With the ongoing reorganisation of the I/O layers, it's visible that cs_recv()
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will have to transfer data between the cs' rxbuf and the channel's buffer while
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not being aware of the data format. Moreover, in case there's no data there, it
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needs to recursively call the mux's rcv_buf() to trigger a decoding, while this
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function is sometimes replaced with cs_recv(). All this shows that cs_recv() is
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in fact needed while data are pushed upstream from the lower layers, and is not
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suitable for the "pull" mode. Thus it was decided to remove this function and
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put its code back into h2_rcv_buf(). The H1 mux's rcv_buf() already couldn't be
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replaced with cs_recv() since it is the only one knowing about the buffer's
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format.
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This opportunity simplified something : if the cs's rxbuf is only read by the
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mux's rcv_buf() method, then it doesn't need to be located into the CS and is
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well placed into the mux's representation of the stream. This has an important
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impact for H2 as it offers more freedom to the mux to allocate/free/reallocate
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this buffer, and it ensures the mux always has access to it.
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Furthermore, the conn_stream's txbuf experienced the same fate. Indeed, the H1
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mux has already uncovered the difficulty related to the channel shutting down
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on output, with data stuck into the CS's txbuf. Since the CS is tightly coupled
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to the stream and the stream can close immediately once its buffers are empty,
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it required a way to support orphaned CS with pending data in their txbuf. This
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is something that the H2 mux already has to deal with, by carefully leaving the
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data in the channel's buffer. But due to the snd_buf() call being top-down, it
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is always possible to push the stream's data via the mux's snd_buf() call
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without requiring a CS txbuf anymore. Thus the txbuf (when needed) is only
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implemented in the mux and attached to the mux's representation of the stream,
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and doing so allows to immediately release the channel once the data are safe
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in the mux's buffer.
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This is an important change which clarifies the roles and responsibilities of
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each layer in the chain : when receiving data from a mux, it's the mux's
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responsibility to make sure it can correctly decode the incoming data and to
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buffer the possible excess of data it cannot pass to the requester. This means
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that decoding an H2 frame, which is not retryable since it has an impact on the
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HPACK decompression context, and which cannot be reordered for the same reason,
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simply needs to be performed to the H2 stream's rxbuf which will then be passed
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to the stream when this one calls h2_rcv_buf(), even if it reads one byte at a
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time. Similarly when calling h2_snd_buf(), it's the mux's responsibility to
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read as much as it needs to be able to restart later, possibly by buffering
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some data into a local buffer. And it's only once all the output data has been
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consumed by snd_buf() that the stream is free to disappear.
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This model presents the nice benefit of being infinitely stackable and solving
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the last identified showstoppers to move towards a structured message internal
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representation, as it will give full power to the rcv_buf() and snd_buf() to
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process what they need.
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For now the conn_stream's flags indicating whether a shutdown has been seen in
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any direction or if an end of stream was seen will remain in the conn_stream,
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though it's likely that some of them will move to the mux's representation of
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the stream after structured messages are implemented.
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