[rabbitmq-discuss] Huge latency in Linux, compared with Leopard

Carl Trieloff cctrieloff at redhat.com
Sun Sep 28 02:19:59 BST 2008

> - kernel settings matter, but not as much as one would think. My server
> runs at 250 HZ & voluntary preemption, whereas the laptop runs with full
> preemption at 300 HZ - however both exhibit very similar symptoms, and I
> strongly suspect you'd see the same at 1000 HZ or with the RT kernel
> (still need to try that one). Keep in mind that different distributions
> have patched kernels to varying degrees (especially RedHat) and that the
> relatively new CFQ CPU scheduler (new in 2.6.23 IIRC) had a lot of
> performance oddities since its introduction. My understanding from
> following the kernel list most of these should be fixed in the current
> 2.6.26 kernel, however the variance between the average (~1ms) and max.
> latency (40ms) is IMHO just way too big for an occasional mis-schedule so
> something else must be wrong. Besides we all get the same 40ms penalty so
> that is a good sign that it's not the kernel scheduler per se.

I'll add one note, the Red Hat MRG RT kernel gives you a max scheduler 
latency of ~20us. The
Linux TCP stack can run with max latency sub 1ms with both 2.6.18 RHEL5 
and MRG RT
2.6.24. most of your issues are most likely nagle.


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