Re: [RFC v2 PATCH 00/17] variable-order, large folios for anonymous memory

Ryan Roberts
2023-04-26T10:41:55Z

Hi David,

On 17/04/2023 16:44, David Hildenbrand wrote:

>>>>>
>>>>> So what should be safe is replacing all sub-pages of a folio that are marked
>>>>> "maybe shared" by a new folio under PT lock. However, I wonder if it's really
>>>>> worth the complexity. For THP we were happy so far to *not* optimize this,
>>>>> implying that maybe we shouldn't worry about optimizing the fork() case for
>>>>> now
>>>>> that heavily.
>>>>
>>>> I don't have the exact numbers to hand, but I'm pretty sure I remember enabling
>>>> large copies was contributing a measurable amount to the performance
>>>> improvement. (Certainly, the zero-page copy case, is definitely a big
>>>> contributer). I don't have access to the HW at the moment but can rerun later
>>>> with and without to double check.
>>>
>>> In which test exactly? Some micro-benchmark?
>>
>> The kernel compile benchmark that I quoted numbers for in the cover letter. I
>> have some trace points (not part of the submitted series) that tell me how many
>> mappings of each order we get for each code path. I'm pretty sure I remember all
>> of these 4 code paths contributing non-negligible amounts.
>
> Interesting! It would be great to see if there is an actual difference after
> patch #10 was applied without the other COW replacement.
>

Sorry about the delay. I now have some numbers for this...

I rearranged the patch order so that all the "utility" stuff (new rmap
functions, etc) are first (1, 2, 3, 4, 5, 8, 9, 11, 12, 13), followed by a
couple of general improvements (7, 17), which should be dormant until we have
the final patches, then finally (6, 10, 14, 15), which implement large anon
folios the allocate, reuse, copy-non-zero and copy-zero paths respectively. I've
dropped patch 16 and fixed the copy-exclusive bug you spotted (by ensuring we
never replace an exclusive page).

I've measured performance at the following locations in the patch set:

- baseline: none of my patches applied
- utility: has utility and general improvement patches applied
- alloc: utility + 6
- reuse: utility + 6 + 10
- copy: utility + 6 + 10 + 14
- zero-alloc: utility + 6 + 19 + 14 + 15

The test is `make defconfig && time make -jN Image` for a clean checkout of
v6.3-rc3. The first result is thrown away, and the next 3 are kept. I saw some
per-boot variance (probably down to kaslr, etc). So have booted each kernel 7
times for a total of 3x7=21 samples per kernel. Then I've taken the mean:

jobs=8:

| label | real | user | kernel |
|:-----------|-------:|-------:|---------:|
| baseline | 0.0% | 0.0% | 0.0% |
| utility | -2.7% | -2.8% | -3.1% |
| alloc | -6.0% | -2.3% | -24.1% |
| reuse | -9.5% | -5.8% | -28.5% |
| copy | -10.6% | -6.9% | -29.4% |
| zero-alloc | -9.2% | -5.1% | -29.8% |

jobs=160:

| label | real | user | kernel |
|:-----------|-------:|-------:|---------:|
| baseline | 0.0% | 0.0% | 0.0% |
| utility | -1.8% | -0.0% | -7.7% |
| alloc | -6.0% | 1.8% | -20.9% |
| reuse | -7.8% | -1.6% | -24.1% |
| copy | -7.8% | -2.5% | -26.8% |
| zero-alloc | -7.7% | 1.5% | -29.4% |

So it looks like patch 10 (reuse) is making a difference, but copy and
zero-alloc are not adding a huge amount, as you hypothesized. Personally I would
prefer not to drop those patches though, as it will all help towards utilization
of contiguous PTEs on arm64, which is the second part of the change that I'm now
working on.

For the final config ("zero-alloc") I also collected stats on how many
operations each of the 4 paths was performing, using ftrace and histograms.
"pnr" is the number of pages allocated/reused/copied, and "fnr" is the number of
pages in the source folio):

do_anonymous_page:

{ pnr: 1 } hitcount: 2749722
{ pnr: 4 } hitcount: 387832
{ pnr: 8 } hitcount: 409628
{ pnr: 16 } hitcount: 4296115

pages: 76315914
faults: 7843297
pages per fault: 9.7

wp_page_reuse (anon):

{ pnr: 1, fnr: 1 } hitcount: 47887
{ pnr: 3, fnr: 4 } hitcount: 2
{ pnr: 4, fnr: 4 } hitcount: 6131
{ pnr: 6, fnr: 8 } hitcount: 1
{ pnr: 7, fnr: 8 } hitcount: 10
{ pnr: 8, fnr: 8 } hitcount: 3794
{ pnr: 1, fnr: 16 } hitcount: 36
{ pnr: 2, fnr: 16 } hitcount: 23
{ pnr: 3, fnr: 16 } hitcount: 5
{ pnr: 4, fnr: 16 } hitcount: 9
{ pnr: 5, fnr: 16 } hitcount: 8
{ pnr: 6, fnr: 16 } hitcount: 9
{ pnr: 7, fnr: 16 } hitcount: 3
{ pnr: 8, fnr: 16 } hitcount: 24
{ pnr: 9, fnr: 16 } hitcount: 2
{ pnr: 10, fnr: 16 } hitcount: 1
{ pnr: 11, fnr: 16 } hitcount: 9
{ pnr: 12, fnr: 16 } hitcount: 2
{ pnr: 13, fnr: 16 } hitcount: 27
{ pnr: 14, fnr: 16 } hitcount: 2
{ pnr: 15, fnr: 16 } hitcount: 54
{ pnr: 16, fnr: 16 } hitcount: 6673

pages: 211393
faults: 64712
pages per fault: 3.3

wp_page_copy (anon):

{ pnr: 1, fnr: 1 } hitcount: 81242
{ pnr: 4, fnr: 4 } hitcount: 5974
{ pnr: 1, fnr: 8 } hitcount: 1
{ pnr: 4, fnr: 8 } hitcount: 1
{ pnr: 8, fnr: 8 } hitcount: 12933
{ pnr: 1, fnr: 16 } hitcount: 19
{ pnr: 4, fnr: 16 } hitcount: 3
{ pnr: 8, fnr: 16 } hitcount: 7
{ pnr: 16, fnr: 16 } hitcount: 4106

pages: 274390
faults: 104286
pages per fault: 2.6

wp_page_copy (zero):

{ pnr: 1 } hitcount: 178699
{ pnr: 4 } hitcount: 14498
{ pnr: 8 } hitcount: 23644
{ pnr: 16 } hitcount: 257940

pages: 4552883
faults: 474781
pages per fault: 9.6

Thanks,
Ryan