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60 lines
2.7 KiB
Plaintext
60 lines
2.7 KiB
Plaintext
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** Plug queueing disciplines **
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The 'plug' qdisc type is not documented. It is even not supported
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by traffic shaping tools like 'tc' from iproute2 package.
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Such qdiscs have already been used by Yelp engineers but outside
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of haproxy with libnl-utils tools (especially nl-qdisc-* tools)
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to implement a workaround and make haproxy reloads work.
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Indeed with such plug qdiscs coupled with iptables configurations
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we are able to temporarily bufferize IP packets and to release them as
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needed. So, they may be very useful to "synchronize" TCP sessions
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or at higher level to put network applications in states approaching
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the ones suspected to occur during bugs. Furthermore to be sure
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to produce a correct bug fix, it may be useful to reproduce
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as mush as needed such painful bugs. This is where plug qdiscs
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may be useful.
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To have an idea about how to use plug qdisc on the command line I highly recommend to
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read Willy Tarreau blog here:
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https://www.haproxy.com/blog/truly-seamless-reloads-with-haproxy-no-more-hacks/
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which refers to this other one from Yelp:
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https://engineeringblog.yelp.com/2015/04/true-zero-downtime-haproxy-reloads.html
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The code found in plug_qdisc.c file already helped in fixing a painful bug hard to
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fix because hard to reproduce. To use the API it exports this is quite easy:
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- First your program must call plug_disc_attach() to create if not already created
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a plug qdisc and use it (must be done during your application own already existing
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initializations).
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Note that this function calls plug_qdisc_release_indefinite_buffer() so that to
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release already buffered packets before you start your application,
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- then call plug_qdisc_plug_buffer() to start buffering packets incoming to your
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plug qdisc. So they won't be delivered to your application,
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- then call plug_qdisc_release_indefinite_buffer() to stop buffering the packets
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incoming to your plug qdisc and release those already buffered.
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So, that to be deliver them to your application.
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This code is short and simple. But uses several libraries especially libnl-route module
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part of libnl library. To compile haproxy and make it use the plug_qdisc.c code we had
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to link it against several libnl3 library modules like that:
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-lnl-genl-3 -lnl-route-3 -lnl-3 -lnl-cli-3
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- Some references:
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Libnl API documentation may be found here:
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https://www.infradead.org/~tgr/libnl/doc/api/index.html
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Kernel sources:
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http://elixir.free-electrons.com/linux/latest/source/net/sched/sch_plug.c
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Nice website about traffic shaping with queuing disciplines:
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http://wiki.linuxwall.info/doku.php/en:ressources:dossiers:networking:traffic_control
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