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I got into an argument on the net.core.somaxconn parameter: I was told that it will not make any difference if we change the default 128.

I believed this might be enough proof:

"If the backlog argument is greater than the value in /proc/sys/net/core/somaxconn, then it is silently truncated to that value" http://linux.die.net/man/2/listen

but it's not.

Does anyone know a method to testify this with two machines, sitting on a Gbit network? The best would be against MySQL, LVS, apache2 ( 2.2 ), memcached.

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1 Answer 1

Setting net.core.somaxconn to values higher than default is only needed on very-very loaded servers where connection rate is so high/bursty that having 128 (in BSDs even more: 128 backlog + 64 half-open) concurrent connections is not considered abnormal or when you need to delegate definition of what is normal to people writing application or it's config.

Some administrators use high net.core.somaxconn to hide problems with their services, so from user's point of view process stall would look like a latency spike instead of connection interrupted/timeout (controlled by net.ipv4.tcp_abort_on_overflow in Linux).

Real cause is either slow processing of some requests (e.g. some single threaded blocking server) or insufficient number of worker threads/processes in software (e.g. multi- process/threaded blocking software like apache)

PS. Also as listen(2) manual says - net.core.somaxconn acts only upper boundary for an application which is free to choose something smaller (usually set in app's config), though some apps just use listen(fd, -1) which means set backlog to the max.

PPS. Sometimes it's preferable to fail fast and let the load-balancer to do it's job than to make user wait - for that purpose we set net.core.somaxconn to some high values like 4096, but limit application backlog to something small like 10 and set net.ipv4.tcp_abort_on_overflow to 1.

PPPS. Many versions of Linux have nasty bug of truncating somaxcon value to it's 16 lower bits (i.e. casting value to uint16_t), so raising that value to more than 65535 can be even dangerous. For more information see: http://patchwork.ozlabs.org/patch/255460/

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