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

[Minchan Kim <minchan.kim@gmail.com>]

On Wed, Jul 20, 2011 at 3:10 PM, Shaohua Li <shaohua.li@intel.com> 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@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

Yes. Sorry for the typo.

> speed, CPU isn't bottleneck here.

But you try to optimize CPU. so your experiment is not good.

>
>> 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 )

-- 
Kind regards,
Minchan Kim

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