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    SubjectRe: [patch v3] swap: virtual swap readahead
    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
    > ?
    >
    > > 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;

    + ptep = pte_offset_map_lock(mm, pmd, min, &ptl);
    + for (i = nr = 0; i < limit; i++)
    + if (is_swap_pte(ptep[i]))
    + entries[nr++] = pte_to_swp_entry(ptep[i]);
    + pte_unmap_unlock(ptep, ptl);

    Cheer!,
    -Kame




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