Re: [patch v3] swap: virtual swap readahead

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Jun 18, 2009 at 05:19:49PM +0800, Wu Fengguang wrote:
> On Tue, Jun 16, 2009 at 02:22:17AM +0800, Johannes Weiner wrote:
> > On Fri, Jun 12, 2009 at 09:59:27AM +0800, Wu Fengguang wrote:
> > > On Thu, Jun 11, 2009 at 06:17:42PM +0800, Johannes Weiner wrote:
> > > > On Thu, Jun 11, 2009 at 01:22:28PM +0800, Wu Fengguang wrote:
> > > > > Unfortunately, after fixing it up the swap readahead patch still performs slow
> > > > > (even worse this time):
> > > > 
> > > > Thanks for doing the tests.  Do you know if the time difference comes
> > > > from IO or CPU time?
> > > > 
> > > > Because one reason I could think of is that the original code walks
> > > > the readaround window in two directions, starting from the target each
> > > > time but immediately stops when it encounters a hole where the new
> > > > code just skips holes but doesn't abort readaround and thus might
> > > > indeed read more slots.
> > > > 
> > > > I have an old patch flying around that changed the physical ra code to
> > > > use a bitmap that is able to represent holes.  If the increased time
> > > > is waiting for IO, I would be interested if that patch has the same
> > > > negative impact.
> > > 
> > > You can send me the patch :)
> > 
> > Okay, attached is a rebase against latest -mmotm.
> > 
> > > But for this patch it is IO bound. The CPU iowait field actually is
> > > going up as the test goes on:
> > 
> > It's probably the larger ra window then which takes away the bandwidth
> > needed to load the new executables.  This sucks.  Would be nice to
> > have 'optional IO' for readahead that is dropped when normal-priority
> > IO requests are coming in...  Oh, we have READA for bios.  But it
> > doesn't seem to implement dropping requests on load (or I am blind).
> 
> Hi Hannes,
> 
> Sorry for the long delay! A bad news is that I get many oom with this patch:

Okay, evaluating this test-patch any further probably isn't worth it.
It's too aggressive, I think readahead is stealing pages reclaimed by
other allocations which in turn oom.

Back to the original problem: you detected increased latency for
launching new applications, so they get less share of the IO bandwidth
than without the patch.

I can see two reasons for this:

  a) the new heuristics don't work out and we read more unrelated
  pages than before

  b) we readahead more pages in total as the old code would stop at
  holes, as described above

We can verify a) by comparing major fault numbers between the two
kernels with your testload.  If they increase with my patch, we
anticipate the wrong slots and every fault has do the reading itself.

b) seems to be a trade-off.  After all, the IO resources you have less
for new applications in your test is the bandwidth that is used by
swapping applications.  My qsbench numbers are a sign for this as the
only IO going on is swap.

Of course, the theory is not to improve swap performance by increasing
the readahead window but to choose better readahead candidates.  So I
will run your tests and qsbench with a smaller page cluster and see if
this improves both loads.

Let me no if that doesn't make sense :)

Thanks a lot for all your efforts so far,

	Hannes

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