Re: [patch v3] swap: virtual swap readahead

[KAMEZAWA Hiroyuki]

On Wed, 10 Jun 2009 16:11:32 +0800
Wu Fengguang <fengguang.wu@xxxxxxxxx> 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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