Re: [PATCH -v2 -mm] add extra free kbytes tunable

[Rik van Riel]

On 10/11/2011 04:54 PM, Andrew Morton wrote:
> On Tue, 11 Oct 2011 16:23:22 -0400
> Satoru Moriya<satoru.moriya@xxxxxxx>  wrote:
>
> > On 10/11/2011 03:55 PM, Andrew Morton wrote:
> > > On Tue, 11 Oct 2011 15:32:11 -0400
> > > Satoru Moriya<satoru.moriya@xxxxxxx>  wrote:
> > >
> > > > On 10/10/2011 06:37 PM, Andrew Morton wrote:
> > > > > On Fri, 7 Oct 2011 20:08:19 -0700 (PDT) David Rientjes
> > > > > <rientjes@xxxxxxxxxx>  wrote:
> > > > >
> > > > > > On Thu, 1 Sep 2011, Rik van Riel wrote:
> > > >
> > > > Actually page allocator decreases min watermark to 3/4 * min
> > > > watermark for rt-task. But in our case some applications create a lot
> > > > of processes and if all of them are rt-task, the amount of watermark
> > > > bonus(1/4 * min watermark) is not enough.
> > > >
> > > > If we can tune the amount of bonus, it may be fine. But that is
> > > > almost all same as extra free kbytes.
> > >
> > > This situation is detectable at runtime.  If realtime tasks are being
> > > stalled in the page allocator then start to increase the free-page
> > > reserves.  A little control system.
> >
> > Detecting at runtime is too late for some latency critical systems.
> > At that system, we must avoid a stall before it happens.
>
> It's pretty darn obvious that the kernel can easily see the situation
> developing before it happens.  By comparing a few integers.

The problem is that we may be dealing with bursts, not steady
states of allocations.  Without knowing the size of a burst,
we have no idea when we should wake up kswapd to get enough
memory freed ahead of the application's allocations.

> Look, please don't go bending over backwards like this to defend a bad
> patch.  It's a bad patch!  It would be better not to have to merge it.
> Let's do something better.

I would love it if we could come up with something better,
and have thought about it a lot.

However, so far we do not seem to have an alternative yet :(

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