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.
>
> > This stupid-waiting can be improved, and I am working on that. But
>
> Yeah, stupid waiting :)
>
> > 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.
>
> > > On Mon, Aug 09, 2010 at 09:30:00AM -0400, Matthew Wilcox wrote:
> > > >
> > > > This testcase shows some odd behaviour from the Linux VM.
> > > >
> > > > It creates a 1TB sparse file, mmaps it, and randomly reads locations
> > > > in it. Due to the file being entirely sparse, the VM allocates new pages
> > > > and zeroes them. Initially, it runs very fast, taking on the order of
> > > > 2.7 to 4us per page fault. Eventually, the VM runs out of free pages,
> > > > and starts doing huge amounts of work trying to figure out which of
> > > > these clean pages to throw away.
> >
> > This is similar to one of my test cases for:
> >
> > 6457474 vmscan: detect mapped file pages used only once
> > 31c0569 vmscan: drop page_mapping_inuse()
> > dfc8d63 vmscan: factor out page reference checks
> >
> > because the situation was even worse before (see the series
> > description in dfc8d63). Maybe asking the obvious, but the kernel you
> > tested on did include those commits, right?
> >
> > And just to be sure, I sent you a test-patch to disable the used-once
> > detection on IRC the other day. Did you have time to run it yet?
> > Here it is again:
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 9c7e57c..c757bba 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -584,6 +584,7 @@ static enum page_references page_check_references(struct page *page,
> > return PAGEREF_RECLAIM;
> >
> > if (referenced_ptes) {
> > + return PAGEREF_ACTIVATE;
>
> How come page activation helps?
>
> > if (PageAnon(page))
> > return PAGEREF_ACTIVATE;
> > /*
> >
> >
> > > > In my testing with a 6GB machine and 2.9GHz CPU, one in every
> > > > 15,000 page faults takes over a second, and one in every 40,000
> > > > page faults take over seven seconds!
> > > >
> > > > This test-case demonstrates a problem that occurs with a read-mostly
> > > > mmap of a file on very fast media. I wouldn't like to see a solution
> > > > that special-cases zeroed pages. I think userspace has done its part
> > > > to tell the kernel what's it's doing by calling madvise(MADV_RANDOM).
> > > > This ought to be enough to hint to the kernel that it should be eagerly
> > > > throwing away pages in this VMA.
> >
> > We can probably do something like the following, but I am not sure
> > this is a good fix, either. How many applications are using
> > madvise()?
>
> Heh, it sounds crazy to rip random read pages, though it does help to
> produce a FAST test case.
>
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -495,7 +495,7 @@ int page_referenced_one(struct page *pag
> > * mapping is already gone, the unmap path will have
> > * set PG_referenced or activated the page.
> > */
> > - if (likely(!VM_SequentialReadHint(vma)))
> > + if (likely(!(vma->vm_flags & (VM_SEQ_READ|VM_RAND_READ))))
> > referenced++;
> > }
>
> Thanks,
> Fengguang
>
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