Re: [PATCH v3 00/15] mm/memory: optimize fork() with PTE-mapped THP

David Hildenbrand
2024-01-31T15:05:08Z

On 31.01.24 16:02, Ryan Roberts wrote:
> On 31/01/2024 14:29, David Hildenbrand wrote:
>>>> Note that regarding NUMA effects, I mean when some memory access within the same
>>>> socket is faster/slower even with only a single node. On AMD EPYC that's
>>>> possible, depending on which core you are running and on which memory controller
>>>> the memory you want to access is located. If both are in different quadrants
>>>> IIUC, the access latency will be different.
>>>
>>> I've configured the NUMA to only bring the RAM and CPUs for a single socket
>>> online, so I shouldn't be seeing any of these effects. Anyway, I've been using
>>> the Altra as a secondary because its so much slower than the M2. Let me move
>>> over to it and see if everything looks more straightforward there.
>>
>> Better use a system where people will actually run Linux production workloads
>> on, even if it is slower :)
>>
>> [...]
>>
>>>>>
>>>>> I'll continue to mess around with it until the end of the day. But I'm not
>>>>> making any headway, then I'll change tack; I'll just measure the performance of
>>>>> my contpte changes using your fork/zap stuff as the baseline and post based on
>>>>> that.
>>>>
>>>> You should likely not focus on M2 results. Just pick a representative bare metal
>>>> machine where you get consistent, explainable results.
>>>>
>>>> Nothing in the code is fine-tuned for a particular architecture so far, only
>>>> order-0 handling is kept separate.
>>>>
>>>> BTW: I see the exact same speedups for dontneed that I see for munmap. For
>>>> example, for order-9, it goes from 0.023412s -> 0.009785, so -58%. So I'm
>>>> curious why you see a speedup for munmap but not for dontneed.
>>>
>>> Ugh... ok, coming up.
>>
>> Hopefully you were just staring at the wrong numbers (e.g., only with fork
>> patches). Because both (munmap/pte-dontneed) are using the exact same code path.
>>
>
> Ahh... I'm doing pte-dontneed, which is the only option in your original
> benchmark - it does MADV_DONTNEED one page at a time. It looks like your new
> benchmark has an additional "dontneed" option that does it in one shot. Which
> option are you running? Assuming the latter, I think that explains it.

I temporarily removed that option and then re-added it. Guess you got a
wrong snapshot of the benchmark :D

pte-dontneed not observing any change is great (no batching possible).

dontneed should hopefully/likely see a speedup.

Great!

--
Cheers,

David / dhildenb