Re: [RFC][PATCH 6/8] mm: handle_speculative_fault()

[Ed Tomlinson]

On Friday 08 January 2010 11:53:30 Peter Zijlstra wrote:
> On Tue, 2010-01-05 at 20:20 -0800, Linus Torvalds wrote:
> > 
> > On Wed, 6 Jan 2010, KAMEZAWA Hiroyuki wrote:
> > > > 
> > > > Of course, your other load with MADV_DONTNEED seems to be horrible, and 
> > > > has some nasty spinlock issues, but that looks like a separate deal (I 
> > > > assume that load is just very hard on the pgtable lock).
> > > 
> > > It's zone->lock, I guess. My test program avoids pgtable lock problem.
> > 
> > Yeah, I should have looked more at your callchain. That's nasty. Much 
> > worse than the per-mm lock. I thought the page buffering would avoid the 
> > zone lock becoming a huge problem, but clearly not in this case.
> 
> Right, so I ran some numbers on a multi-socket (2) machine as well:
> 
>                                pf/min
> 
> -tip                          56398626
> -tip + xadd                  174753190
> -tip + speculative           189274319
> -tip + xadd + speculative    200174641

Has anyone tried these patches with ramzswap?  Nitin do they help with the locking
issues you mentioned?

Thanks,
Ed

 
> [ variance is around 0.5% for this workload, ran most of these numbers
> with --repeat 5 ]
> 
> At both the xadd/speculative point the workload is dominated by the
> zone->lock, the xadd+speculative removes some of the contention, and
> removing the various RSS counters could yield another few percent
> according to the profiles, but then we're pretty much there.
> 
> One way around those RSS counters is to track it per task, a quick grep
> shows its only the oom-killer and proc that use them.
> 
> A quick hack removing them gets us: 203158058
> 
> So from a throughput pov. the whole speculative fault thing might not be
> interesting until the rest of the vm gets a lift to go along with it.
> 
> >From a blocking on mmap_sem pov. I think Linus is right in that we
> should first consider things like dropping mmap_sep around IO and page
> zeroing, and generally looking at reducing hold times and such.
> 
> So while I think its quite feasible to do these speculative faults, it
> appears we're not quite ready for them.
> 
> Maybe I can get -rt to carry it for a while, there we have to reduce
> mmap_sem to a mutex, which hurts lots.
> 
> 
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