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Date: Wed, 31 Jan 2024 16:05:01 +0100
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Subject: Re: [PATCH v3 00/15] mm/memory: optimize fork() with PTE-mapped THP
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To: Ryan Roberts <ryan.roberts@arm.com>, linux-kernel@vger.kernel.org
Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
 Matthew Wilcox <willy@infradead.org>, Russell King <linux@armlinux.org.uk>,
 Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org>,
 Dinh Nguyen <dinguyen@kernel.org>, Michael Ellerman <mpe@ellerman.id.au>,
 Nicholas Piggin <npiggin@gmail.com>,
 Christophe Leroy <christophe.leroy@csgroup.eu>,
 "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>,
 "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>,
 Paul Walmsley <paul.walmsley@sifive.com>, Palmer Dabbelt
 <palmer@dabbelt.com>, Albert Ou <aou@eecs.berkeley.edu>,
 Alexander Gordeev <agordeev@linux.ibm.com>,
 Gerald Schaefer <gerald.schaefer@linux.ibm.com>,
 Heiko Carstens <hca@linux.ibm.com>, Vasily Gorbik <gor@linux.ibm.com>,
 Christian Borntraeger <borntraeger@linux.ibm.com>,
 Sven Schnelle <svens@linux.ibm.com>, "David S. Miller"
 <davem@davemloft.net>, linux-arm-kernel@lists.infradead.org,
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From: David Hildenbrand <david@redhat.com>
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In-Reply-To: <a34eee7e-3970-4cdd-8c09-bca51132db50@arm.com>
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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