From: Ryan Roberts <ryan.roberts@arm.com>
To: David Hildenbrand <david@redhat.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,
linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org,
linux-s390@vger.kernel.org, sparclinux@vger.kernel.org
Subject: Re: [PATCH v3 00/15] mm/memory: optimize fork() with PTE-mapped THP
Date: Wed, 31 Jan 2024 13:16:48 +0000 [thread overview]
Message-ID: <de975655-8f8f-40dc-b281-75c40dd1e2c1@arm.com> (raw)
In-Reply-To: <30718fc8-15cf-41e4-922c-5cdbf00a0840@redhat.com>
On 31/01/2024 12:56, David Hildenbrand wrote:
> On 31.01.24 13:37, Ryan Roberts wrote:
>> On 31/01/2024 11:49, Ryan Roberts wrote:
>>> On 31/01/2024 11:28, David Hildenbrand wrote:
>>>> On 31.01.24 12:16, Ryan Roberts wrote:
>>>>> On 31/01/2024 11:06, David Hildenbrand wrote:
>>>>>> On 31.01.24 11:43, Ryan Roberts wrote:
>>>>>>> On 29/01/2024 12:46, David Hildenbrand wrote:
>>>>>>>> Now that the rmap overhaul[1] is upstream that provides a clean interface
>>>>>>>> for rmap batching, let's implement PTE batching during fork when processing
>>>>>>>> PTE-mapped THPs.
>>>>>>>>
>>>>>>>> This series is partially based on Ryan's previous work[2] to implement
>>>>>>>> cont-pte support on arm64, but its a complete rewrite based on [1] to
>>>>>>>> optimize all architectures independent of any such PTE bits, and to
>>>>>>>> use the new rmap batching functions that simplify the code and prepare
>>>>>>>> for further rmap accounting changes.
>>>>>>>>
>>>>>>>> We collect consecutive PTEs that map consecutive pages of the same large
>>>>>>>> folio, making sure that the other PTE bits are compatible, and (a) adjust
>>>>>>>> the refcount only once per batch, (b) call rmap handling functions only
>>>>>>>> once per batch and (c) perform batch PTE setting/updates.
>>>>>>>>
>>>>>>>> While this series should be beneficial for adding cont-pte support on
>>>>>>>> ARM64[2], it's one of the requirements for maintaining a total mapcount[3]
>>>>>>>> for large folios with minimal added overhead and further changes[4] that
>>>>>>>> build up on top of the total mapcount.
>>>>>>>>
>>>>>>>> Independent of all that, this series results in a speedup during fork with
>>>>>>>> PTE-mapped THP, which is the default with THPs that are smaller than a PMD
>>>>>>>> (for example, 16KiB to 1024KiB mTHPs for anonymous memory[5]).
>>>>>>>>
>>>>>>>> On an Intel Xeon Silver 4210R CPU, fork'ing with 1GiB of PTE-mapped folios
>>>>>>>> of the same size (stddev < 1%) results in the following runtimes
>>>>>>>> for fork() (shorter is better):
>>>>>>>>
>>>>>>>> Folio Size | v6.8-rc1 | New | Change
>>>>>>>> ------------------------------------------
>>>>>>>> 4KiB | 0.014328 | 0.014035 | - 2%
>>>>>>>> 16KiB | 0.014263 | 0.01196 | -16%
>>>>>>>> 32KiB | 0.014334 | 0.01094 | -24%
>>>>>>>> 64KiB | 0.014046 | 0.010444 | -26%
>>>>>>>> 128KiB | 0.014011 | 0.010063 | -28%
>>>>>>>> 256KiB | 0.013993 | 0.009938 | -29%
>>>>>>>> 512KiB | 0.013983 | 0.00985 | -30%
>>>>>>>> 1024KiB | 0.013986 | 0.00982 | -30%
>>>>>>>> 2048KiB | 0.014305 | 0.010076 | -30%
>>>>>>>
>>>>>>> Just a heads up that I'm seeing some strange results on Apple M2. Fork for
>>>>>>> order-0 is seemingly costing ~17% more. I'm using GCC 13.2 and was pretty
>>>>>>> sure I
>>>>>>> didn't see this problem with version 1; although that was on a different
>>>>>>> baseline and I've thrown the numbers away so will rerun and try to debug
>>>>>>> this.
>>>
>>> Numbers for v1 of the series, both on top of 6.8-rc1 and rebased to the same
>>> mm-unstable base as v3 of the series (first 2 rows are from what I just posted
>>> for context):
>>>
>>> | kernel | mean_rel | std_rel |
>>> |:-------------------|-----------:|----------:|
>>> | mm-unstabe (base) | 0.0% | 1.1% |
>>> | mm-unstable + v3 | 16.7% | 0.8% |
>>> | mm-unstable + v1 | -2.5% | 1.7% |
>>> | v6.8-rc1 + v1 | -6.6% | 1.1% |
>>>
>>> So all looks good with v1. And seems to suggest mm-unstable has regressed by ~4%
>>> vs v6.8-rc1. Is this really a useful benchmark? Does the raw performance of
>>> fork() syscall really matter? Evidence suggests its moving all over the place -
>>> breath on the code and it changes - not a great place to be when using the test
>>> for gating purposes!
>>>
>>> Still with the old tests - I'll move to the new ones now.
>>>
>>>
>>>>>>>
>>>>>>
>>>>>> So far, on my x86 tests (Intel, AMD EPYC), I was not able to observe this.
>>>>>> fork() for order-0 was consistently effectively unchanged. Do you observe
>>>>>> that
>>>>>> on other ARM systems as well?
>>>>>
>>>>> Nope; running the exact same kernel binary and user space on Altra, I see
>>>>> sensible numbers;
>>>>>
>>>>> fork order-0: -1.3%
>>>>> fork order-9: -7.6%
>>>>> dontneed order-0: -0.5%
>>>>> dontneed order-9: 0.1%
>>>>> munmap order-0: 0.0%
>>>>> munmap order-9: -67.9%
>>>>>
>>>>> So I guess some pipelining issue that causes the M2 to stall more?
>>>>
>>>> With one effective added folio_test_large(), it could only be a code layout
>>>> problem? Or the compiler does something stupid, but you say that you run the
>>>> exact same kernel binary, so that doesn't make sense.
>>>
>>> Yup, same binary. We know this code is very sensitive - 1 cycle makes a big
>>> difference. So could easily be code layout, branch prediction, etc...
>>>
>>>>
>>>> I'm also surprised about the dontneed vs. munmap numbers.
>>>
>>> You mean the ones for Altra that I posted? (I didn't post any for M2). The altra
>>> numbers look ok to me; dontneed has no change, and munmap has no change for
>>> order-0 and is massively improved for order-9.
>>>
>>> Doesn't make any sense
>>>> (again, there was this VMA merging problem but it would still allow for
>>>> batching
>>>> within a single VMA that spans exactly one large folio).
>>>>
>>>> What are you using as baseline? Really just mm-unstable vs.
>>>> mm-unstable+patches?
>>>
>>> yes. except for "v6.8-rc1 + v1" above.
>>>
>>>>
>>>> Let's see if the new test changes the numbers you measure.
>>
>> Nope: looks the same. I've taken my test harness out of the picture and done
>> everything manually from the ground up, with the old tests and the new. Headline
>> is that I see similar numbers from both.
>
> I took me a while to get really reproducible numbers on Intel. Most importantly:
> * Set a fixed CPU frequency, disabling any boost and avoiding any
> thermal throttling.
> * Pin the test to CPUs and set a nice level.
I'm already pinning the test to cpu 0. But for M2, at least, I'm running in a VM
on top of macos, and I don't have a mechanism to pin the QEMU threads to the
physical CPUs. Anyway, I don't think these are problems because for a given
kernel build I can accurately repro numbers.
>
> Another thing is, to avoid systems where you can have NUMA effects within a
> single socket. Otherwise, memory access latency is just random and depends on
> what the buddy enjoys giving you.
Yep; same. M2 is 1 NUMA node. On Altra, I'm disabling the second NUMA node to
remove those effects.
>
> But you seem to get the same +17 even after reboots, so that indicates that the
> CPU is not happy about the code for some reason. And the weird thing is, that
> nothing significantly changed for order-0 folios between v1 and v3 that could
> explain any of this.
>
> I'm not worried about 5% or so, nobody cares. But it would be good to have at
> least an explanation why only that system shows +17%.
Yep understood.
>
>>
>> Some details:
>> - I'm running for 10 seconds then averaging the output
>
> Same here.
>
>> - test is bimodal; first run (of 10 seconds) after boot is a bit faster on
>> average (up to 10%) than the rest; I could guess this is due to the memory
>> being allocated more contiguously the first few times through, so struct
>> pages have better locality, but that's a guess.
>
> I think it also has to do with the PCP lists, and the high-pcp auto tuning (I
> played with disabling that). Running on a freshly booted system gave me
> reproducible results.
>
> But yes: I was observing something similar on AMD EPYC, where you get
> consecutive pages from the buddy, but once you allocate from the PCP it might no
> longer be consecutive.
>
>> - test is 5-10% slower when output is printed to terminal vs when redirected to
>> file. I've always effectively been redirecting. Not sure if this overhead
>> could start to dominate the regression and that's why you don't see it?
>
> That's weird, because we don't print while measuring? Anyhow, 5/10% variance on
> some system is not the end of the world.
I imagine its cache effects? More work to do to print the output could be
evicting some code that's in the benchmark path?
>
>>
>> I'm inclined to run this test for the last N kernel releases and if the number
>> moves around significantly, conclude that these tests don't really matter.
>> Otherwise its an exercise in randomly refactoring code until it works well, but
>> that's just overfitting to the compiler and hw. What do you think?
>
> Personally, I wouldn't lose sleep if you see weird, unexplainable behavior on
> some system (not even architecture!). Trying to optimize for that would indeed
> be random refactorings.
>
> But I would not be so fast to say that "these tests don't really matter" and
> then go wild and degrade them as much as you want. There are use cases that care
> about fork performance especially with order-0 pages -- such as Redis.
Indeed. But also remember that my fork baseline time is ~2.5ms, and I think you
said yours was 14ms :)
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.
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next prev parent reply other threads:[~2024-01-31 13:17 UTC|newest]
Thread overview: 48+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-01-29 12:46 [PATCH v3 00/15] mm/memory: optimize fork() with PTE-mapped THP David Hildenbrand
2024-01-29 12:46 ` [PATCH v3 01/15] arm64/mm: Make set_ptes() robust when OAs cross 48-bit boundary David Hildenbrand
2024-02-08 6:10 ` Mike Rapoport
2024-02-09 22:36 ` David Hildenbrand
2024-01-29 12:46 ` [PATCH v3 02/15] arm/pgtable: define PFN_PTE_SHIFT David Hildenbrand
2024-02-08 6:11 ` Mike Rapoport
2024-01-29 12:46 ` [PATCH v3 03/15] nios2/pgtable: " David Hildenbrand
2024-02-08 6:12 ` Mike Rapoport
2024-01-29 12:46 ` [PATCH v3 04/15] powerpc/pgtable: " David Hildenbrand
2024-02-08 6:13 ` Mike Rapoport
2024-01-29 12:46 ` [PATCH v3 05/15] riscv/pgtable: " David Hildenbrand
2024-02-08 6:14 ` Mike Rapoport
2024-01-29 12:46 ` [PATCH v3 06/15] s390/pgtable: " David Hildenbrand
2024-02-08 6:15 ` Mike Rapoport
2024-01-29 12:46 ` [PATCH v3 07/15] sparc/pgtable: " David Hildenbrand
2024-02-08 6:18 ` Mike Rapoport
2024-01-29 12:46 ` [PATCH v3 08/15] mm/pgtable: make pte_next_pfn() independent of set_ptes() David Hildenbrand
2024-02-08 6:19 ` Mike Rapoport
2024-01-29 12:46 ` [PATCH v3 09/15] arm/mm: use pte_next_pfn() in set_ptes() David Hildenbrand
2024-02-08 6:20 ` Mike Rapoport
2024-01-29 12:46 ` [PATCH v3 10/15] powerpc/mm: " David Hildenbrand
2024-02-08 6:20 ` Mike Rapoport
2024-01-29 12:46 ` [PATCH v3 11/15] mm/memory: factor out copying the actual PTE in copy_present_pte() David Hildenbrand
2024-02-08 6:29 ` Mike Rapoport
2024-01-29 12:46 ` [PATCH v3 12/15] mm/memory: pass PTE to copy_present_pte() David Hildenbrand
2024-02-08 6:27 ` Mike Rapoport
2024-02-14 22:40 ` David Hildenbrand
2024-01-29 12:46 ` [PATCH v3 13/15] mm/memory: optimize fork() with PTE-mapped THP David Hildenbrand
2024-02-08 6:41 ` Mike Rapoport
2024-01-29 12:46 ` [PATCH v3 14/15] mm/memory: ignore dirty/accessed/soft-dirty bits in folio_pte_batch() David Hildenbrand
2024-01-29 12:46 ` [PATCH v3 15/15] mm/memory: ignore writable bit " David Hildenbrand
2024-01-31 10:43 ` [PATCH v3 00/15] mm/memory: optimize fork() with PTE-mapped THP Ryan Roberts
2024-01-31 11:06 ` David Hildenbrand
2024-01-31 11:16 ` Ryan Roberts
2024-01-31 11:28 ` David Hildenbrand
2024-01-31 11:49 ` Ryan Roberts
2024-01-31 12:37 ` Ryan Roberts
2024-01-31 12:56 ` David Hildenbrand
2024-01-31 13:16 ` Ryan Roberts [this message]
2024-01-31 13:38 ` David Hildenbrand
2024-01-31 13:58 ` Ryan Roberts
2024-01-31 14:29 ` David Hildenbrand
2024-01-31 15:02 ` Ryan Roberts
2024-01-31 15:05 ` David Hildenbrand
2024-01-31 15:08 ` Ryan Roberts
2024-01-31 15:11 ` David Hildenbrand
2024-01-31 12:59 ` David Hildenbrand
2024-03-25 4:42 ` patchwork-bot+linux-riscv
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