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

[Shaohua Li]

On Wed, 2011-07-20 at 14:30 +0800, Minchan Kim wrote:
> On Wed, Jul 20, 2011 at 3:10 PM, Shaohua Li <shaohua.li@xxxxxxxxx> wrote:
> > 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@xxxxxxxxx> 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
> 
> Yes. Sorry for the typo.
> 
> > speed, CPU isn't bottleneck here.
> 
> But you try to optimize CPU. so your experiment is not good.
it's not that good, because the disk isn't fast. The swap test is the
workload with most significant impact I can get.

> >> 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.
> 
> If it begin swapping, I think the the performance would be less important,
> But your patch is so simple that it would be mergable(Maybe Andrew
> would merge regardless of my comment) but impact is a little in your
> experiment.
> 
> I suggest you test it with fast disk like SSD and show the benefit to
> us certainly. (I think you intel guy have a good SSD, apparently :D )
I do have one, but it's not good :). The high throughput is just like a
normal disk. A hardware raid is preferred to do the test, but I haven't.

Thanks,
Shaohua

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