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Message-ID: <87r024o1zt.fsf@DESKTOP-5N7EMDA>
Date: Mon, 07 Apr 2025 15:38:14 +0800
From: "Huang, Ying" <ying.huang@...ux.alibaba.com>
To: Nikhil Dhama <nikhil.dhama@....com>
Cc: <akpm@...ux-foundation.org>, <bharata@....com>,
<huang.ying.caritas@...il.com>, <linux-kernel@...r.kernel.org>,
<linux-mm@...ck.org>, <mgorman@...hsingularity.net>,
<raghavendra.kodsarathimmappa@....com>, <oe-lkp@...ts.linux.dev>,
<lkp@...el.com>
Subject: Re: [PATCH] mm: pcp: scale batch to reduce number of high order pcp
flushes on deallocation
Nikhil Dhama <nikhil.dhama@....com> writes:
> On 4/3/2025 7:06 AM, Huang, Ying wrote:
>
>>
>> Nikhil Dhama <nikhil.dhama@....com> writes:
>>
>>> On 3/30/2025 12:22 PM, Huang, Ying wrote:
>>>
>>>>
>>>> Hi, Nikhil,
>>>>
>>>> Nikhil Dhama <nikhil.dhama@....com> writes:
>>>>
>>>>> In old pcp design, pcp->free_factor gets incremented in nr_pcp_free()
>>>>> which is invoked by free_pcppages_bulk(). So, it used to increase
>>>>> free_factor by 1 only when we try to reduce the size of pcp list or
>>>>> flush for high order.
>>>>> and free_high used to trigger only for order > 0 and order <
>>>>> costly_order and free_factor > 0.
>>>>>
>>>>> and free_factor used to scale down by a factor of 2 on every successful
>>>>> allocation.
>>>>>
>>>>> for iperf3 I noticed that with older design in kernel v6.6, pcp list was
>>>>> drained mostly when pcp->count > high (more often when count goes above
>>>>> 530). and most of the time free_factor was 0, triggering very few
>>>>> high order flushes.
>>>>>
>>>>> Whereas in the current design, free_factor is changed to free_count to keep
>>>>> track of the number of pages freed contiguously,
>>>>> and with this design for iperf3, pcp list is getting flushed more
>>>>> frequently because free_high heuristics is triggered more often now.
>>>>>
>>>>> In current design, free_count is incremented on every deallocation,
>>>>> irrespective of whether pcp list was reduced or not. And logic to
>>>>> trigger free_high is if free_count goes above batch (which is 63) and
>>>>> there are two contiguous page free without any allocation.
>>>>> (and with cache slice optimisation).
>>>>>
>>>>> With this design, I observed that high order pcp list is drained as soon
>>>>> as both count and free_count goes about 63.
>>>>>
>>>>> and due to this more aggressive high order flushing, applications
>>>>> doing contiguous high order allocation will require to go to global list
>>>>> more frequently.
>>>>>
>>>>> On a 2-node AMD machine with 384 vCPUs on each node,
>>>>> connected via Mellonox connectX-7, I am seeing a ~30% performance
>>>>> reduction if we scale number of iperf3 client/server pairs from 32 to 64.
>>>>>
>>>>> So, though this new design reduced the time to detect high order flushes,
>>>>> but for application which are allocating high order pages more
>>>>> frequently it may be flushing the high order list pre-maturely.
>>>>> This motivates towards tuning on how late or early we should flush
>>>>> high order lists.
>>>>>
>>>>> for free_high heuristics. I tried to scale batch and tune it,
>>>>> which will delay the free_high flushes.
>>>>>
>>>>>
>>>>> score # free_high
>>>>> ----------- ----- -----------
>>>>> v6.6 (base) 100 4
>>>>> v6.12 (batch*1) 69 170
>>>>> batch*2 69 150
>>>>> batch*4 74 101
>>>>> batch*5 100 53
>>>>> batch*6 100 36
>>>>> batch*8 100 3
>>>>>
>>>>> scaling batch for free_high heuristics with a factor of 5 or above restores
>>>>> the performance, as it is reducing the number of high order flushes.
>>>>>
>>>>> On 2-node AMD server with 384 vCPUs each,score for other benchmarks with
>>>>> patch v2 along with iperf3 are as follows:
>>>>
>>>> Em..., IIUC, this may disable the free_high optimizations. free_high
>>>> optimization is introduced by Mel Gorman in commit f26b3fa04611
>>>> ("mm/page_alloc: limit number of high-order pages on PCP during bulk
>>>> free"). So, this may trigger regression for the workloads in the
>>>> commit. Can you try it too?
>>>>
>>>
>>> Hi, I ran netperf-tcp as in commit f26b3fa04611 ("mm/page_alloc: limit
>>> number of high-order pages on PCP during bulk free"),
>>>
>>> On a 2-node AMD server with 384 vCPUs, results I observed are as follows:
>>>
>>> 6.12 6.12
>>> vanilla freehigh-heuristicsopt
>>> Hmean 64 732.14 ( 0.00%) 736.90 ( 0.65%)
>>> Hmean 128 1417.46 ( 0.00%) 1421.54 ( 0.29%)
>>> Hmean 256 2679.67 ( 0.00%) 2689.68 ( 0.37%)
>>> Hmean 1024 8328.52 ( 0.00%) 8413.94 ( 1.03%)
>>> Hmean 2048 12716.98 ( 0.00%) 12838.94 ( 0.96%)
>>> Hmean 3312 15787.79 ( 0.00%) 15822.40 ( 0.22%)
>>> Hmean 4096 17311.91 ( 0.00%) 17328.74 ( 0.10%)
>>> Hmean 8192 20310.73 ( 0.00%) 20447.12 ( 0.67%)
>>>
>>> It is not regressing for netperf-tcp.
>>
>> Thanks a lot for your data!
>>
>> Think about this again. Compared with the pcp->free_factor solution,
>> the pcp->free_count solution will trigger free_high heuristics more
>> early, this causes performance regression in your workloads. So, it's
>> reasonable to raise the bar to trigger free_high. And, it's also
>> reasonable to use a stricter threshold, as you have done in this patch.
>> However, "5 * batch" appears too magic and adapt to one type of machine.
>>
>> Let's step back to do some analysis. In the original pcp->free_factor
>> solution, free_high is triggered for contiguous freeing with size
>> ranging from "batch" to "pcp->high + batch". So, the average value is
>> about "batch + pcp->high / 2". While in the pcp->free_count solution,
>> free_high will be triggered for contiguous freeing with size "batch".
>> So, to restore the original behavior, it seems that we can use the
>> threshold "batch + pcp->high_min / 2". Do you think that this is
>> reasonable? If so, can you give it a try?
>
> Hi,
>
> I have tried your suggestion as setting threshold to "batch + pcp->high_min / 2",
> scores for different benchmarks on the same machine
> (2-Node AMD server with 384 vCPUs each) are as follows:
>
> iperf3 lmbench3 netperf kbuild
> (AF_UNIX) (SCTP_STREAM_MANY)
> ------- --------- ----------------- ------
> v6.6 vanilla (base) 100 100 100 100
> v6.12 vanilla 69 113 98.5 98.8
> v6.12 avg_threshold 100 110.3 100.2 99.3
>
> and for netperf-tcp, it is as follows:
>
> 6.12 6.12
> vanilla avg_free_high_threshold
> Hmean 64 732.14 ( 0.00%) 730.45 ( -0.23%)
> Hmean 128 1417.46 ( 0.00%) 1419.44 ( 0.14%)
> Hmean 256 2679.67 ( 0.00%) 2676.45 ( -0.12%)
> Hmean 1024 8328.52 ( 0.00%) 8339.34 ( 0.13%)
> Hmean 2048 12716.98 ( 0.00%) 12743.68 ( 0.21%)
> Hmean 3312 15787.79 ( 0.00%) 15887.25 ( 0.63%)
> Hmean 4096 17311.91 ( 0.00%) 17332.68 ( 0.12%)
> Hmean 8192 20310.73 ( 0.00%) 20465.09 ( 0.76%)
Thanks a lot for test and results!
It looks good to me. Can you submit a formal patch?
---
Best Regards,
Huang, Ying
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