Re: [PATCH] mm: scale kswapd watermarks in proportion to memory

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Feb 18, 2016 at 03:15:43PM -0500, Rik van Riel wrote:
> On Thu, 2016-02-18 at 11:41 -0500, Johannes Weiner wrote:
> > In machines with 140G of memory and enterprise flash storage, we have
> > seen read and write bursts routinely exceed the kswapd watermarks and
> > cause thundering herds in direct reclaim. Unfortunately, the only way
> > to tune kswapd aggressiveness is through adjusting min_free_kbytes -
> > the system's emergency reserves - which is entirely unrelated to the
> > system's latency requirements. In order to get kswapd to maintain a
> > 250M buffer of free memory, the emergency reserves need to be set to
> > 1G. That is a lot of memory wasted for no good reason.
> > 
> > On the other hand, it's reasonable to assume that allocation bursts
> > and overall allocation concurrency scale with memory capacity, so it
> > makes sense to make kswapd aggressiveness a function of that as well.
> > 
> > Change the kswapd watermark scale factor from the currently fixed 25%
> > of the tunable emergency reserve to a tunable 0.001% of memory.
> > 
> > On a 140G machine, this raises the default watermark steps - the
> > distance between min and low, and low and high - from 16M to 143M.
> 
> This is an excellent idea for a large system,
> but your patch reduces the gap between watermarks
> on small systems.
> 
> On an 8GB zone, your patch halves the gap between
> the watermarks, and on smaller systems it would be
> even worse.

You're right, I'll address that in v2.

> Would it make sense to keep using the old calculation
> on small systems, when the result of the old calculation
> exceeds that of the new calculation?
> 
> Using the max of the two calculations could prevent
> the issue you are trying to prevent on large systems,
> from happening on smaller systems.

Yes, I think enforcing a reasonable minimum this way makes sense.

Thanks Rik.