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

["Huang, Ying" <ying.huang@linux.alibaba.com>]

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?

---
Best Regards,
Huang, Ying