Re: Dirty pages underflow on 3.14.23

[Simon Kirby <sim@hostway.ca>]

On Wed, Jan 07, 2015 at 10:48:10PM +0100, Vlastimil Babka wrote:

> On 01/07/2015 10:28 PM, Simon Kirby wrote:
>
> > Hmm...A possibly-related issue...Before trying this, after a fresh boot,
> > /proc/vmstat showed:
> > 
> > nr_alloc_batch 4294541205
> 
> This can happen, and not be a problem in general. However, there was a fix
> abe5f972912d086c080be4bde67750630b6fb38b in 3.17 for a potential performance
> issue if this counter overflows on single processor configuration. It was marked
> stable, but the 3.16 series was discontinued before the fix could be backported.
> So if you are on single-core, you might hit the performance issue.

That particular commit seems to just change the code path in that case,
but should it be underflowing at all on UP?

> > Still, nr_alloc_batch reads as 4294254379 after MySQL restart, and now
> > seems to stay up there.
> 
> Hm if it stays there, then you are probably hitting the performance issue. Look
> at /proc/zoneinfo, which zone has the underflow. It means this zone will get
> unfair amount of allocations, while others may contain stale data and would be
> better candidates.

In this case, it has only 640MB, and there's only DMA and Normal. This is
affecting Normal, and DMA is so small that it probably doesn't matter.

Simon-

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