Re: [experimental][PATCH] mm,vmstat: per cpu stat flush too when per cpu page cache flushed

[Christoph Lameter <cl@linux.com>]

On Thu, 14 Oct 2010, KOSAKI Motohiro wrote:

> Initial variable ZVC commit (df9ecaba3f1) says
>
> >     [PATCH] ZVC: Scale thresholds depending on the size of the system
> >
> >     The ZVC counter update threshold is currently set to a fixed value of 32.
> >     This patch sets up the threshold depending on the number of processors and
> >     the sizes of the zones in the system.
> >
> >     With the current threshold of 32, I was able to observe slight contention
> >     when more than 130-140 processors concurrently updated the counters.  The
> >     contention vanished when I either increased the threshold to 64 or used
> >     Andrew's idea of overstepping the interval (see ZVC overstep patch).
> >
> >     However, we saw contention again at 220-230 processors.  So we need higher
> >     values for larger systems.
>
> So, I'm worry about your patch reintroduce old cache contention issue that Christoph
> observed when run 128-256cpus system.  May I ask how do you think this issue?

The load that I ran with was a test that concurrently faulted pages on a
large number of processors. This is a bit artificial and is only of
performance concern during startup of a large HPC job. The frequency of
counter updates during regular processing should pose a much lighter load
on the system. The automatic adaption of the thresholds should

1. Preserve the initial startup performance (since the threshold will be
unmodified on a system just starting).

2. Reduce the overhead of establish a more accurate zone state (because
reclaim can then cause the thresholds to be adapted).


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