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 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?

---
Best Regards,
Huang, Ying