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Message-ID: <20250407063259.49271-1-nikhil.dhama@amd.com>
Date: Mon, 7 Apr 2025 12:02:59 +0530
From: Nikhil Dhama <nikhil.dhama@....com>
To: <ying.huang@...ux.alibaba.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>, Nikhil Dhama <nikhil.dhama@....com>
Subject: Re: [PATCH] mm: pcp: scale batch to reduce number of high order pcp flushes on deallocation
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,
Nikhil Dhama
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