Re: [PATCH v4 2/5] mm: LARGE_ANON_FOLIO for improved performance

[Yin Fengwei]

On 8/3/23 17:32, Ryan Roberts wrote:
> On 03/08/2023 09:37, Yin Fengwei wrote:
>>
>>
>> On 8/3/23 16:21, Ryan Roberts wrote:
>>> On 03/08/2023 09:05, Yin Fengwei wrote:
>>>
>>> ...
>>>
>>>>> I've captured run time and peak memory usage, and taken the mean. The stdev for
>>>>> the peak memory usage is big-ish, but I'm confident this still captures the
>>>>> central tendancy well:
>>>>>
>>>>> | MAX_ORDER_UNHINTED |   real-time |   kern-time |   user-time | peak memory |
>>>>> |:-------------------|------------:|------------:|------------:|:------------|
>>>>> | 4k                 |        0.0% |        0.0% |        0.0% |        0.0% |
>>>>> | 16k                |       -3.6% |      -26.5% |       -0.5% |       -0.1% |
>>>>> | 32k                |       -4.8% |      -37.4% |       -0.6% |       -0.1% |
>>>>> | 64k                |       -5.7% |      -42.0% |       -0.6% |       -1.1% |
>>>>> | 128k               |       -5.6% |      -42.1% |       -0.7% |        1.4% |
>>>>> | 256k               |       -4.9% |      -41.9% |       -0.4% |        1.9% |
>>>>
>>>> Here is my test result:
>>>>
>>>> 		real		user		sys
>>>> hink-4k:	 0%		0%		0%
>>>> hink-16K:	-3%		0.1%		-18.3%
>>>> hink-32K:	-4%		0.2%		-27.2%
>>>> hink-64K:	-4%		0.5%		-31.0%
>>>> hink-128K:	-4%		0.9%		-33.7%
>>>> hink-256K:	-5%		1%		-34.6%
>>>>
>>>>
>>>> I used command: 
>>>> /usr/bin/time -f "\t%E real,\t%U user,\t%S sys" make -skj96 allmodconfig all
>>>> to build kernel and collect the real time/user time/kernel time.
>>>> /sys/kernel/mm/transparent_hugepage/enabled is "madvise".
>>>> Let me know if you have any question about the test.
>>>
>>> Thanks for doing this! I have a couple of questions:
>>>
>>>  - how many times did you run each test?
>>      Three times for each ANON_FOLIO_MAX_ORDER_UNHINTED. The stddev is quite
>>      small like less than %1.
> 
> And out of interest, were you running on bare metal or in VM? And did you reboot
> between each run?
I run the test on bare metal env. I didn't reboot for every run. But have to reboot
for different ANON_FOLIO_MAX_ORDER_UNHINTED size. I do
   echo 3 > /proc/sys/vm/drop_caches
for everything run after "make mrproper" even after a fresh boot.


> 
>>>
>>>  - how did you configure the large page size? (I sent an email out yesterday
>>>    saying that I was doing it wrong from my tests, so the 128k and 256k results
>>>    for my test set are not valid.
>>      I changed the ANON_FOLIO_MAX_ORDER_UNHINTED definition manually every time.
> 
> In that case, I think your results are broken in a similar way to mine. This
> code means that order will never be higher than 3 (32K) on x86:
> 
> +		order = max(arch_wants_pte_order(), PAGE_ALLOC_COSTLY_ORDER);
> +
> +		if (!hugepage_vma_check(vma, vma->vm_flags, false, true, true))
> +			order = min(order, ANON_FOLIO_MAX_ORDER_UNHINTED);
> 
> On x86, arch_wants_pte_order() is not implemented and the default returns -1, so
> you end up with:
I added arch_waits_pte_order() for x86 and gave it a very large number. So the
order is decided by ANON_FOLIO_MAX_ORDER_UNHINTED. I suppose my data is valid.

> 
> 	order = min(PAGE_ALLOC_COSTLY_ORDER, ANON_FOLIO_MAX_ORDER_UNHINTED)
> 
> So your 4k, 16k and 32k results should be valid, but 64k, 128k and 256k results
> are actually using 32k, I think? Which is odd because you are getting more
> stddev than the < 1% you quoted above? So perhaps this is down to rebooting
> (kaslr, or something...?)
> 
> (on arm64, arch_wants_pte_order() returns 4, so my 64k result is also valid).
> 
> As a quick hack to work around this, would you be able to change the code to this:
> 
> +		if (!hugepage_vma_check(vma, vma->vm_flags, false, true, true))
> +			order = ANON_FOLIO_MAX_ORDER_UNHINTED;
> 
>>
>>>
>>>  - what does "hink" mean??
>>      Sorry for the typo. It should be ANON_FOLIO_MAX_ORDER_UNHINTED.
>>
>>>
>>>>
>>>> I also find one strange behavior with this version. It's related with why
>>>> I need to set the /sys/kernel/mm/transparent_hugepage/enabled to "madvise".
>>>> If it's "never", the large folio is disabled either.
>>>> If it's "always", the THP will be active before large folio. So the system is
>>>> in the mixed mode. it's not suitable for this test.
>>>
>>> We had a discussion around this in the THP meeting yesterday. I'm going to write
>>> this up propoerly so we can have proper systematic discussion. The tentative
>>> conclusion is that MADV_NOHUGEPAGE must continue to mean "do not fault in more
>>> than is absolutely necessary". I would assume we need to extend that thinking to
>>> the process-wide and system-wide knobs (as is done in the patch), but we didn't
>>> explicitly say so in the meeting.
>> There are cases that THP is not appreciated because of the latency or memory
>> consumption. For these cases, large folio may fill the gap as less latency and
>> memory consumption.
>>
>>
>> So if disabling THP means large folio can't be used, we loose the chance to
>> benefit those cases with large folio.
> 
> Yes, I appreciate that. But there are also real use cases that expect
> MADV_NOHUGEPAGE means "do not fault more than is absolutely necessary" and the
> use cases break if that's not obeyed (e.g. live migration w/ qemu). So I think
> we need to be conservitive to start. These apps that are explicitly forbidding
> THP today, should be updated in the long run to opt-in to large anon folios
> using some as-yet undefined control.
Fair enough.


Regards
Yin, Fengwei

> 
>>
>>
>> Regards
>> Yin, Fengwei
>>
>>>
>>> My intention is that if you have requested THP and your vma is big enough for
>>> PMD-size then you get that, else you fallback to large anon folios. And if you
>>> have neither opted in nor out, then you get large anon folios.
>>>
>>> We talked about the idea of adding a new knob that let's you set the max order,
>>> but that needs a lot more thought.
>>>
>>> Anyway, as I said, I'll write it up so we can all systematically discuss.
>>>
>>>>
>>>> So if it's "never", large folio is disabled. But why "madvise" enables large
>>>> folio unconditionly? Suppose it's only enabled for the VMA range which user
>>>> madvise large folio (or THP)?
>>>>
>>>> Specific for the hink setting, my understand is that we can't choose it only
>>>> by this testing. Other workloads may have different behavior with differnt
>>>> hink setting.
>>>>
>>>>
>>>> Regards
>>>> Yin, Fengwei
>>>>
>>>
>