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

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Date: Wed, 31 Jan 2024 15:08:58 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
 Matthew Wilcox <willy@...radead.org>, Russell King <linux@...linux.org.uk>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Dinh Nguyen <dinguyen@...nel.org>, Michael Ellerman <mpe@...erman.id.au>,
 Nicholas Piggin <npiggin@...il.com>,
 Christophe Leroy <christophe.leroy@...roup.eu>,
 "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
 "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
 Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt
 <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
 Alexander Gordeev <agordeev@...ux.ibm.com>,
 Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
 Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
 Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Sven Schnelle <svens@...ux.ibm.com>, "David S. Miller"
 <davem@...emloft.net>, linux-arm-kernel@...ts.infradead.org,
 linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
 linux-s390@...r.kernel.org, sparclinux@...r.kernel.org
Subject: Re: [PATCH v3 00/15] mm/memory: optimize fork() with PTE-mapped THP

On 31/01/2024 15:05, David Hildenbrand wrote:
> 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).

indeed.

> 
> dontneed should hopefully/likely see a speedup.

Yes, but that's almost exactly the same path as munmap, so I'm sure it really
adds much for this particular series. Anyway, on Altra at least, I'm seeing no
regressions, so:

Tested-by: Ryan Roberts <ryan.roberts@....com>

> 
> Great!
>