Re: [PATCH]vmscan: add block plug for page reclaim

[Shaohua Li <shaohua.li@intel.com>]

On Wed, 2011-07-20 at 13:53 +0800, Minchan Kim wrote:
> On Wed, Jul 20, 2011 at 11:53 AM, Shaohua Li <shaohua.li@intel.com> wrote:
> > per-task block plug can reduce block queue lock contention and increase request
> > merge. Currently page reclaim doesn't support it. I originally thought page
> > reclaim doesn't need it, because kswapd thread count is limited and file cache
> > write is done at flusher mostly.
> > When I test a workload with heavy swap in a 4-node machine, each CPU is doing
> > direct page reclaim and swap. This causes block queue lock contention. In my
> > test, without below patch, the CPU utilization is about 2% ~ 7%. With the
> > patch, the CPU utilization is about 1% ~ 3%. Disk throughput isn't changed.
> 
> Why doesn't it enhance through?
throughput? The disk isn't that fast. We already can make it run in full
speed, CPU isn't bottleneck here.

> It means merge is rare?
Merge is still there even without my patch, but maybe not be able to
make the request size biggest in cocurrent I/O.

> > This should improve normal kswapd write and file cache write too (increase
> > request merge for example), but might not be so obvious as I explain above.
> 
> CPU utilization enhance on  4-node machine with heavy swap?
> I think it isn't common situation.
> 
> And I don't want to add new stack usage if it doesn't have a benefit.
> As you know, direct reclaim path has a stack overflow.
> These days, Mel, Dave and Christoph try to remove write path in
> reclaim for solving stack usage and enhance write performance.
it will use a little stack, yes. When I said the benefit isn't so
obvious, it doesn't mean it has no benefit. For example, if kswapd and
other threads write the same disk, this can still reduce lock contention
and increase request merge. Part reason I didn't see obvious affect for
file cache is my disk is slow.

Thanks,
Shaohua

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