Re: [gentoo-server] Forthcoming 2.6.13 timer frequency changes & server performance

[Bastian Balthazar Bux]

Kerin Millar wrote:
> Hi folks.
>
> The forthcoming kernel will be introducing a rather visible (and
> potentially important) change. Firstly, it will provide an option to
> select the default timer frequency. Secondly, the default frequency
> will now be 250HZ. For the record, the value used to be 100HZ and was
> rather drastically changed to 1000HZ for 2.6. It will now be possible
> to choose a HZ value of either 100, 250 or 1000.
>
> The most interesting aspect of the timer is that it affects scheduling
> granularity. That is, the minimum possible time for which a given
> process is allowed to run before interruption. This is commonly known
> as a "jiffy" and can be calculated by dividing 1000 by the HZ value.
> So, the length of a jiffy in 2.6 at present is 1ms. In general, a
> longer jiffy results in more throughput but with more latency whereas
> a shorter jiffy results in less throughput (due to the overhead from
> frequent interruption) but with a reduction in latency. In my view,
> this is rather interesting in terms of the impact this can have on
> server performance. The help text for the new kernel configuration
> option explains further:
>
> - "Allows the configuration of the timer frequency. It is customary to
> have the timer interrupt run at 1000 HZ but 100 HZ may be more
> beneficial for servers and NUMA systems that do not need to have a
> fast response for user interaction and that may experience bus
> contention and cacheline bounces as a result of timer interrupts. Note
> that the timer interrupt occurs on each processor in an SMP
> environment leading to NR_CPUS * HZ number of timer interrupts per
> second."
>
> The specific help text for the 100HZ option is as follows:
>
> - "100 HZ is a typical choice for servers, SMP and NUMA systems with
> lots of processors that may show reduced performance if too many timer
> interrupts are occurring."
>
> For 250HZ:
>
> - "250 HZ is a good compromise choice allowing server performance
> while also showing good interactive responsiveness even on SMP and
> NUMA systems."
>
> For 1000HZ:
>
> - "1000 HZ is the preferred choice for desktop systems and other
> systems requiring fast interactive responses to events."
>
> Another interesting point that is not mentioned above is that reducing
> the timer frequency can significantly help to reduce time drift
> (particularly on "real" SMP systems where this effect may be even more
> pronounced). The reason that I am posting is twofold:
>
> * To notify everyone that this change is coming
> * To provide a means for anyone interested to test the ramifications
> of this newly acquired flexibility using a 2.6.12 kernel prior to the
> eventual release of 2.6.13.
>
> To that end I have grabbed the specific patch that adds the
> configuration option which can be found here:
>
>   http://www.recruit2recruit.net/kerframil/2.6.12-i386-selectable-hz.patch
>
> This patch is taken directly from upstream (the option itself can be
> found at the bottom of the "Processor type and features" menu).
> Interestingly, the change has drawn attention to a few areas in the
> kernel where the methods used for timing (such as delay loops) are not
> ideal. So I have gone through all of the patches committed to the
> mainline tree since 2.6.12.3 and collected the ones that are relevant
> then aggregated them into one patch (the second one applies cleanly
> against a gentoo-sources tree):
>
>   http://www.recruit2recruit.net/kerframil/2.6.12-timing-fixes-rollup.patch
>   http://www.recruit2recruit.net/kerframil/2.6.12-gentoo-r7-timing-fixes-rollup.patch
>
> The individual patches are all very small and non-intrusive by nature.
> They are certainly not critical in order to be able to change the
> timer frequency but I recommend that they be used. For the curious,
> some notes on the contents of the patch can be found here here:
> http://www.recruit2recruit.net/kerframil/NOTES-2.6.12-timing-fixes-rollup
>
> What I would like is for anyone who is interested to put an alternate
> value (100 or 250) to the test (if they are in a position to be able
> to do so) and to determine whether there is any clear performance
> improvement with their workload. I switched my system (Compaq Proliant
> ML370) to 100HZ 2 days ago and can at least confirm that it is stable
> although I have not yet had the opportunity to perform any comparative
> benchmarks. If in doubt, then 250 might be a good value to try as,
> after all, that is going to be the default - at least for x86 ;)
>
> Finally, this topic has generated a nice little flame war over on the
> LKML (actually, it's quite an interesting thread if a little confusing
> at points):
>
> http://kerneltrap.org/node/5430
> http://lkml.org/lkml/2005/7/8/259
>
> Cheers,
>
> --Kerin Millar
>

Good idea to publicize this.

Another thing that should be mentioned is that the timer frequency for
2.4 kernels was(/is?) fixed to 100 Hz.

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