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

From: Jens Axboe
Date: Sat Jul 23 2011 - 14:52:01 EST

On 2011-07-22 07:14, Shaohua Li wrote:
> On Fri, 2011-07-22 at 03:32 +0800, Jens Axboe wrote:
>> On 2011-07-20 08:49, Shaohua Li wrote:
>>> On Wed, 2011-07-20 at 14:30 +0800, Minchan Kim wrote:
>>>> On Wed, Jul 20, 2011 at 3:10 PM, Shaohua Li <> wrote:
>>>>> On Wed, 2011-07-20 at 13:53 +0800, Minchan Kim wrote:
>>>>>> On Wed, Jul 20, 2011 at 11:53 AM, Shaohua Li <> 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.
>> Let me just interject here that a plug should be fine, from 3.1 we'll
>> even auto-unplug if a certain depth has been reached. So latency should
>> not be a worry. Personally I think the patch looks fine, though some
>> numbers would be interesting to see. Cycles spent submitting the actual
>> IO, combined with IO statistics what kind of IO patterns were observed
>> for plain and with patch would be good.
> I can observe the average request size changes. Before the patch, the
> average request size is about 90k from iostat (but the variation is
> big). With the patch, the request size is about 100k and variation is
> small.

That's a good win right there, imho.

> how to check the cycles spend submitting the I/O?

It's not easy, normal profiles is pretty much the only thing to go by.

I guess I'm just a bit puzzled by Minchan's reluctance towards the
patch, it seems like mostly goodness to me.

Jens Axboe

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