Re: [TESTCASE] Clean pages clogging the VM

Shaohua Li
2010-08-20T05:05:28Z

On Thu, Aug 19, 2010 at 07:51:06PM +0800, Johannes Weiner wrote:
> On Thu, Aug 19, 2010 at 12:07:31AM +0800, Wu Fengguang wrote:
> > On Thu, Aug 19, 2010 at 12:06:13AM +0800, Wu Fengguang wrote:
> > > On Wed, Aug 18, 2010 at 04:13:08PM +0200, Johannes Weiner wrote:
> > > > Hi Matthew,
> > > >
> > > > On Tue, Aug 17, 2010 at 03:50:01PM -0400, Matthew Wilcox wrote:
> > > > >
> > > > > No comment on this? Was it just that I posted it during the VM summit?
> > > >
> > > > I have not forgotten about it. I just have a hard time reproducing
> > > > those extreme stalls you observed.
> > > >
> > > > Running that test on a 2.5GHz machine with 2G of memory gives me
> > > > stalls of up to half a second. The patchset I am experimenting with
> > > > gets me down to peaks of 70ms, but it needs further work.
> > > >
> > > > Mapped file pages get two rounds on the LRU list, so once the VM
> > > > starts scanning, it has to go through all of them twice and can only
> > > > reclaim them on the second encounter.
> > > >
> > > > At that point, since we scan without making progress, we start waiting
> > > > for IO, which is not happening in this case, so we sit there until a
> > > > timeout expires.
> > >
> > > Right, this could lead to some 1s stall. Shaohua and me also noticed
> > > this when investigating the responsiveness issues. And we are wondering
> > > if it makes sense to do congestion_wait() only when the bdi is really
> > > congested? There are no IO underway anyway in this case.
>
> I am currently trying to get rid of all the congestion_wait() in the VM.
> They are used for different purposes, so they need different replacement
> mechanisms.
>
> I saw Shaohua's patch to make congestion_wait() cleverer. But I really
> think that congestion is not a good predicate in the first place. Why
> would the VM care about IO _congestion_? It needs a bunch of pages to
> complete IO, whether the writing device is congested is not really
> useful information at this point, I think.
>
> > > > since I can not reproduce your observations, I don't know if this is
> > > > the (sole) source of the problem. Can I send you patches?
> > >
> > > Sure.
>
> Cool!
congestion_wait() isn't the sole source in my test.
with congestion_wait() removed, the max latency is ~50ms.
while if I made the mmaped page reclaimed in one round (makes page_check_references
return PAGEREF_RECLAIM_CLEAN for mmaped pages) in the test, the max latency is ~150us.

Thanks,
Shaohua