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

Ryan Roberts
2024-01-31T11:49:31Z

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.
>