Re: [mm/page_alloc] f26b3fa046: netperf.Throughput_Mbps -18.0% regression

[Aaron Lu]

On 5/11/2022 3:32 PM, ying.huang@xxxxxxxxx wrote:
> On Wed, 2022-05-11 at 11:40 +0800, Aaron Lu wrote:
>> On Tue, May 10, 2022 at 02:23:28PM +0800, ying.huang@xxxxxxxxx wrote:
>>> On Tue, 2022-05-10 at 11:43 +0800, Aaron Lu wrote:
>>>> On 5/7/2022 3:44 PM, ying.huang@xxxxxxxxx wrote:
>>>>> On Sat, 2022-05-07 at 15:31 +0800, Aaron Lu wrote:
>>>>
>>>> ... ...
>>>>
>>>>>>
>>>>>> I thought the overhead of changing the cache line from "shared" to
>>>>>> "own"/"modify" is pretty cheap.
>>>>>
>>>>> This is the read/write pattern of cache ping-pong.  Although it should
>>>>> be cheaper than the write/write pattern of cache ping-pong in theory, we
>>>>> have gotten sevious regression for that before.
>>>>>
>>>>
>>>> Can you point me to the regression report? I would like to take a look,
>>>> thanks.
>>>
>>> Sure.
>>>
>>> https://lore.kernel.org/all/1425108604.10337.84.camel@xxxxxxxxxxxxxxx/
>>>
>>>>>> Also, this is the same case as the Skylake desktop machine, why it is a
>>>>>> gain there but a loss here? 
>>>>>
>>>>> I guess the reason is the private cache size.  The size of the private
>>>>> L2 cache of SKL server is much larger than that of SKL client (1MB vs.
>>>>> 256KB).  So there's much more core-2-core traffic on SKL server.
>>>>>
>>>>
>>>> It could be. The 256KiB L2 in Skylake desktop can only store 8 order-3
>>>> pages and that means the allocator side may have a higher chance of
>>>> reusing a page that is evicted from the free cpu's L2 cache than the
>>>> server machine, whose L2 can store 40 order-3 pages.
>>>>
>>>> I can do more tests using different high for the two machines:
>>>> 1) high=0, this is the case when page reuse is the extreme. core-2-core
>>>> transfer should be the most. This is the behavior of this bisected commit.
>>>> 2) high=L2_size, this is the case when page reuse is fewer compared to
>>>> the above case, core-2-core should still be the majority.
>>>> 3) high=2 times of L2_size and smaller than llc size, this is the case
>>>> when cache reuse is further reduced, and when the page is indeed reused,
>>>> it shouldn't cause core-2-core transfer but can benefit from llc.
>>>> 4) high>llc_size, this is the case when page reuse is the least and when
>>>> page is indeed reused, it is likely not in the entire cache hierarchy.
>>>> This is the behavior of this bisected commit's parent commit for the
>>>> Skylake desktop machine.
>>>>
>>>> I expect case 3) should give us the best performance and 1) or 4) is the
>>>> worst for this testcase.
>>>>
>>>> case 4) is difficult to test on the server machine due to the cap of
>>>> pcp->high which is affected by the low watermark of the zone. The server
>>>> machine has 128 cpus but only 128G memory, which makes the pcp->high
>>>> capped at 421, while llc size is 40MiB and that translates to a page
>>>> number of 12288.
>>>>>
>>>
>>> Sounds good to me.
>>
>> I've run the tests on a 2 sockets Icelake server and a Skylake desktop.
>>
>> On this 2 sockets Icelake server(1.25MiB L2 = 320 pages, 48MiB LLC =
>> 12288 pages):
>>
>> pcp->high      score
>>     0          100662 (bypass PCP, most page resue, most core-2-core transfer)
>>   320(L2)      117252
>>   640          133149
>>  6144(1/2 llc) 134674
>> 12416(>llc)    103193 (least page reuse)
>>
>> Setting pcp->high to 640(2 times L2 size) gives very good result, only
>> slightly lower than 6144(1/2 llc size). Bypassing PCP to get the most
>> cache reuse didn't deliver good performance, so I think Ying is right:
>> core-2-core really hurts.
>>
>> On this 4core/8cpu Skylake desktop(256KiB L2 = 64 pages, 8MiB LLC = 2048
>> pages):
>>
>>    0           86780 (bypass PCP, most page reuse, most core-2-core transfer)
>>   64(L2)       85813
>>  128           85521
>> 1024(1/2 llc)  85557
>> 2176(> llc)    74458 (least page reuse)
>>
>> Things are different on this small machine. Bypassing PCP gives the best
>> performance. I find it hard to explain this. Maybe the 256KiB is too
>> small that even bypassing PCP, the page still ends up being evicted from
>> L2 when allocator side reuses it? Or maybe core-2-core transfer is
>> fast on this small machine?
> 
> 86780 / 85813 = 1.011
> 
> So, there's almost no measurable difference among the configurations
> except the last one.  I would rather say the test isn't sensitive to L2
> size, but sensitive to LLC size on this machine.
>

Well, if core-2-core transfer is bad for performance, I expect the
performance number of pcp->high=0 to be worse than pcp->high=64 and
pcp->high=128, not as good or even better, that's what I find hard to
explain.

As for performance number being bad when pcp->high > llc, that's
understandable because there is least page/cache reuse and this is the
same for both the desktop machine and that server machine.