Re: [PATCH] mm: pcp: scale batch to reduce number of high order pcp flushes on deallocation

[Nikhil Dhama <nikdhama@amd.com>]

On 4/7/2025 1:08 PM, Huang, Ying wrote:
> Nikhil Dhama <nikhil.dhama@amd.com> writes:
>
>> On 4/3/2025 7:06 AM, Huang, Ying wrote:
>>
>>> Nikhil Dhama <nikhil.dhama@amd.com> writes:
>>>
>>>> On 3/30/2025 12:22 PM, Huang, Ying wrote:
>>>>
>>>>> Hi, Nikhil,
>>>>>
>>>>> Nikhil Dhama <nikhil.dhama@amd.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?

Thank you Huang Ying,  Yes,  I have submitted a formal patch with this.
Patch v3: 
https://lore.kernel.org/linux-mm/20250407105219.55351-1-nikhil.dhama@amd.com/
---
Thanks,
Nikhil Dhama