Re: [patch v3] swap: virtual swap readahead

Johannes Weiner
2009-06-10T09:32:41Z

On Wed, Jun 10, 2009 at 05:32:49PM +0900, KAMEZAWA Hiroyuki wrote:
> On Wed, 10 Jun 2009 16:11:32 +0800
> Wu Fengguang <fengguang.wu@intel.com> wrote:
>
> > On Wed, Jun 10, 2009 at 03:45:08PM +0800, Johannes Weiner wrote:
> > > Hi Fengguang,
> > >
> > > On Wed, Jun 10, 2009 at 01:03:42PM +0800, Wu Fengguang wrote:
> > > > On Wed, Jun 10, 2009 at 03:37:02AM +0800, Johannes Weiner wrote:
> > > > > On Tue, Jun 09, 2009 at 09:01:28PM +0200, Johannes Weiner wrote:
> > > > > > [resend with lists cc'd, sorry]
> > > > >
> > > > > [and fixed Hugh's email. crap]
> > > > >
> > > > > > Hi,
> > > > > >
> > > > > > here is a new iteration of the virtual swap readahead. Per Hugh's
> > > > > > suggestion, I moved the pte collecting to the callsite and thus out
> > > > > > ouf swap code. Unfortunately, I had to bound page_cluster due to an
> > > > > > array of that many swap entries on the stack, but I think it is better
> > > > > > to limit the cluster size to a sane maximum than using dynamic
> > > > > > allocation for this purpose.
> > > >
> > > > Hi Johannes,
> > > >
> > > > When stress testing your patch, I found it triggered many OOM kills.
> > > > Around the time of last OOMs, the memory usage is:
> > > >
> > > > total used free shared buffers cached
> > > > Mem: 474 468 5 0 0 239
> > > > -/+ buffers/cache: 229 244
> > > > Swap: 1023 221 802
> > >
> > > Wow, that really confused me for a second as we shouldn't read more
> > > pages ahead than without the patch, probably even less under stress.
> >
> > Yup - swap readahead is much more challenging than sequential readahead,
> > in that it must be accurate enough given some really obscure patterns.
> >
> > > So the problem has to be a runaway reading. And indeed, severe
> > > stupidity here:
> > >
> > > + window = cluster << PAGE_SHIFT;
> > > + min = addr & ~(window - 1);
> > > + max = min + cluster;
> > > + /*
> > > + * To keep the locking/highpte mapping simple, stay
> > > + * within the PTE range of one PMD entry.
> > > + */
> > > + limit = addr & PMD_MASK;
> > > + if (limit > min)
> > > + min = limit;
> > > + limit = pmd_addr_end(addr, max);
> > > + if (limit < max)
> > > + max = limit;
> > > + limit = max - min;
> > >
> > > The mistake is at the initial calculation of max. It should be
> > >
> > > max = min + window;
> > >
> > > The resulting problem is that min could get bigger than max when
> > > cluster is bigger than PMD_SHIFT. Did you use page_cluster == 5?
> >
> > No I use the default 3.
> >
> > btw, the mistake reflects bad named variables. How about rename
> > cluster => pages
> > window => bytes
> > ?

Proven twice, fixed in v4.

> > > The initial min is aligned to a value below the PMD boundary and max
> > > based on it with a too small offset, staying below the PMD boundary as
> > > well. When min is rounded up, this becomes a bit large:
> > >
> > > limit = max - min;
> > >
> > > So if my brain is already functioning, fixing the initial max should
> > > be enough because either
> > >
> > > o window is smaller than PMD_SIZE, than we won't round down
> > > below a PMD boundary in the first place or
> > >
> > > o window is bigger than PMD_SIZE, than we can round down below
> > > a PMD boundary but adding window to that is garuanteed to
> > > cross the boundary again
> > >
> > > and thus max is always bigger than min.
> > >
> > > Fengguang, does this make sense? If so, the patch below should fix
> > > it.
> >
> > Too bad, a quick test of the below patch freezes the box..
> >
>
> + window = cluster << PAGE_SHIFT;
> + min = addr & ~(window - 1);
> + max = min + cluster;
>
> max = min + window; # this is fixed. then,
>
> + /*
> + * To keep the locking/highpte mapping simple, stay
> + * within the PTE range of one PMD entry.
> + */
> + limit = addr & PMD_MASK;
> + if (limit > min)
> + min = limit;
> + limit = pmd_addr_end(addr, max);
> + if (limit < max)
> + max = limit;
> + limit = max - min;
>
> limit = (max - min) >> PAGE_SHIFT;

Head -> desk.

Fixed in v4, thank you.

Hannes