This function will be used to perform CRC32 computations. This one wa
loosely inspired from crc32b found here, and focuses on size and speed
at the same time :
http://www.hackersdelight.org/hdcodetxt/crc.c.txt
Much faster table-based versions exist but are pointless for our usage
here, this hash already sustains gigabit speed which is far faster than
what we'd ever need. Better preserve the CPU's cache instead.
When we were generating a hash, it was done using an unsigned long. When the hash was used
to select a backend, it was sent as an unsigned int. This made it difficult to predict which
backend would be selected.
This patch updates get_hash, and the hash methods to use an unsigned int, to remain consistent
throughout the codebase.
This fix should be backported to 1.5 and probably in part to 1.4.
This function was designed for haproxy while testing other functions
in the past. Initially it was not planned to be used given the not
very interesting numbers it showed on real URL data : it is not as
smooth as the other ones. But later tests showed that the other ones
are extremely sensible to the server count and the type of input data,
especially DJB2 which must not be used on numeric input. So in fact
this function is still a generally average performer and it can make
sense to merge it in the end, as it can provide an alternative to
sdbm+avalanche or djb2+avalanche for consistent hashing or when hashing
on numeric data such as a source IP address or a visitor identifier in
a URL parameter.
Summary:
In testing at tumblr, we found that using djb2 hashing instead of the
default sdbm hashing resulted is better workload distribution to our backends.
This commit implements a change, that allows the user to specify the hash
function they want to use. It does not limit itself to consistent hashing
scenarios.
The supported hash functions are sdbm (default), and djb2.
For a discussion of the feature and analysis, see mailing list thread
"Consistent hashing alternative to sdbm" :
http://marc.info/?l=haproxy&m=138213693909219
Note: This change does NOT make changes to new features, for instance,
applying an avalance hashing always being performed before applying
consistent hashing.
Willy Tarreau
Dan Dubovik
Bhaskar