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

Ryan Roberts
2024-01-31T15:03:02Z

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.