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

[Mel Gorman <mel@csn.ul.ie>]

On Thu, Oct 14, 2010 at 11:50:28AM +0900, KOSAKI Motohiro wrote:
> > On Wed, Oct 13, 2010 at 04:10:43PM +0900, KOSAKI Motohiro wrote:
> > > When memory shortage, we are using drain_pages() for flushing per cpu
> > > page cache. In this case, per cpu stat should be flushed too. because
> > > now we are under memory shortage and we need to know exact free pages.
> > > 
> > > Otherwise get_page_from_freelist() may fail even though pcp was flushed.
> > > 
> > 
> > With my patch adjusting the threshold to a small value while kswapd is awake,
> > it seems less necessary. 
> 
> I agree this.
> 
> > It's also very hard to predict the performance of
> > this. We are certainly going to take a hit to do the flush but we *might*
> > gain slightly if an allocation succeeds because a watermark check passed
> > when the counters were updated. It's a definite hit for a possible gain
> > though which is not a great trade-off. Would need some performance testing.
> > 
> > I still think my patch on adjusting thresholds is our best proposal so
> > far on how to reduce Shaohua's performance problems while still being
> > safer from livelocks due to memory exhaustion.
> 
> OK, I will try to explain a detai of my worry.
> 
> 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?
> 

It only reintroduces the overhead while kswapd is awake and the system is in danger
of accidentally allocating all of its pages. Yes, it's slower but it's
less risky.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

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