Re: [PATCH-RFC] cfq: Disable low_latency by default for 2.6.32

[Mike Galbraith <efault@gmx.de>]

On Thu, 2009-11-26 at 14:20 +0100, Bartlomiej Zolnierkiewicz wrote:
> On Thursday 26 November 2009 02:08:57 pm Mike Galbraith wrote:
> > On Thu, 2009-11-26 at 12:19 +0000, Mel Gorman wrote:
> > > (cc'ing the people from the page allocator failure thread as this might be
> > > relevant to some of their problems)
> > > 
> > > I know this is very last minute but I believe we should consider disabling
> > > the "low_latency" tunable for block devices by default for 2.6.32.  There was
> > > evidence that low_latency was a problem last week for page allocation failure
> > > reports but the reproduction-case was unusual and involved high-order atomic
> > > allocations in low-memory conditions. It took another few days to accurately
> > > show the problem for more normal workloads and it's a bit more wide-spread
> > > than just allocation failures.
> > > 
> > > Basically, low_latency looks great as long as you have plenty of memory
> > > but in low memory situations, it appears to cause problems that manifest
> > > as reduced performance, desktop stalls and in some cases, page allocation
> > > failures. I think most kernel developers are not seeing the problem as they
> > > tend to test on beefier machines and without hitting swap or low-memory
> > > situations for the most part. When they are hitting low-memory situations,
> > > it tends to be for stress tests where stalls and low performance are expected.
> > 
> > Ouch.  It was bad desktop stalls under heavy write that kicked the whole
> > thing off.
> 
> The problem is that 'desktop' means different things for different people
> (for some kernel developers 'desktop' is more like 'a workstation' and for
> others it is more like 'an embedded device').

The stalls I'm talking about were reported for garden variety desktop
PC.  I reproduced them on my supermarket special Q6600 desktop PC.  That
problem has been with us roughly forever, but I'd hoped it had been
cured.  Guess not.

As an idle speculation, I wonder if the sync vs async slice ratios may
not have been knocked out of kilter a bit by giving more to sync.

	-Mike

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