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. >
