Messages in this thread | | | From | David Hildenbrand <> | Subject | Re: [PATCH RFC] mm/madvise: introduce MADV_POPULATE to prefault/prealloc memory | Date | Thu, 25 Feb 2021 09:41:07 +0100 |
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On 24.02.21 15:25, David Hildenbrand wrote: >> + tmp_end = min_t(unsigned long, end, vma->vm_end); >> + pages = populate_vma_page_range(vma, start, tmp_end, &locked); >> + if (!locked) { >> + mmap_read_lock(mm); >> + *prev = NULL; >> + vma = NULL; > > ^ locked = 1; is missing here. > > > --- Simple benchmark --- > > I implemented MADV_POPULATE_READ and MADV_POPULATE_WRITE and performed > some simple measurements to simulate memory preallocation with empty files: > > 1) mmap a 2 MiB/128 MiB/4 GiB region (anonymous, memfd, memfd hugetlb) > 2) Discard all memory using fallocate/madvise > 3) Prefault memory using different approaches and measure the time this > takes. > > I repeat 2)+3) 10 times and compute the average. I only use a single thread. > > Read: Read from each page a byte. > Write: Write one byte of each page (0). > Read/Write: Read one byte and write the value back for each page > POPULATE: MADV_POPULATE (this patch) > POPULATE_READ: MADV_POPULATE_READ > POPULATE_WRITE: MADV_POPULATE_WRITE > > --- Benchmark results --- > > Measuring 10 iterations each: > ================================================== > 2 MiB MAP_PRIVATE: > ************************************************** > Anonymous : Read : 0.159 ms > Anonymous : Write : 0.244 ms > Anonymous : Read+Write : 0.383 ms > Anonymous : POPULATE : 0.167 ms > Anonymous : POPULATE_READ : 0.064 ms > Anonymous : POPULATE_WRITE : 0.165 ms > Memfd 4 KiB : Read : 0.401 ms > Memfd 4 KiB : Write : 0.056 ms > Memfd 4 KiB : Read+Write : 0.075 ms > Memfd 4 KiB : POPULATE : 0.057 ms > Memfd 4 KiB : POPULATE_READ : 0.337 ms > Memfd 4 KiB : POPULATE_WRITE : 0.056 ms > Memfd 2 MiB : Read : 0.041 ms > Memfd 2 MiB : Write : 0.030 ms > Memfd 2 MiB : Read+Write : 0.031 ms > Memfd 2 MiB : POPULATE : 0.031 ms > Memfd 2 MiB : POPULATE_READ : 0.031 ms > Memfd 2 MiB : POPULATE_WRITE : 0.031 ms > ************************************************** > 2 MiB MAP_SHARED: > ************************************************** > Anonymous : Read : 0.071 ms > Anonymous : Write : 0.181 ms > Anonymous : Read+Write : 0.081 ms > Anonymous : POPULATE : 0.069 ms > Anonymous : POPULATE_READ : 0.069 ms > Anonymous : POPULATE_WRITE : 0.115 ms > Memfd 4 KiB : Read : 0.401 ms > Memfd 4 KiB : Write : 0.351 ms > Memfd 4 KiB : Read+Write : 0.414 ms > Memfd 4 KiB : POPULATE : 0.338 ms > Memfd 4 KiB : POPULATE_READ : 0.339 ms > Memfd 4 KiB : POPULATE_WRITE : 0.279 ms > Memfd 2 MiB : Read : 0.031 ms > Memfd 2 MiB : Write : 0.031 ms > Memfd 2 MiB : Read+Write : 0.031 ms > Memfd 2 MiB : POPULATE : 0.031 ms > Memfd 2 MiB : POPULATE_READ : 0.031 ms > Memfd 2 MiB : POPULATE_WRITE : 0.031 ms > ************************************************** > 128 MiB MAP_PRIVATE: > ************************************************** > Anonymous : Read : 7.517 ms > Anonymous : Write : 22.503 ms > Anonymous : Read+Write : 33.186 ms > Anonymous : POPULATE : 18.381 ms > Anonymous : POPULATE_READ : 3.952 ms > Anonymous : POPULATE_WRITE : 18.354 ms > Memfd 4 KiB : Read : 34.300 ms > Memfd 4 KiB : Write : 4.659 ms > Memfd 4 KiB : Read+Write : 6.531 ms > Memfd 4 KiB : POPULATE : 5.219 ms > Memfd 4 KiB : POPULATE_READ : 29.744 ms > Memfd 4 KiB : POPULATE_WRITE : 5.244 ms > Memfd 2 MiB : Read : 10.228 ms > Memfd 2 MiB : Write : 10.130 ms > Memfd 2 MiB : Read+Write : 10.190 ms > Memfd 2 MiB : POPULATE : 10.007 ms > Memfd 2 MiB : POPULATE_READ : 10.008 ms > Memfd 2 MiB : POPULATE_WRITE : 10.010 ms > ************************************************** > 128 MiB MAP_SHARED: > ************************************************** > Anonymous : Read : 7.295 ms > Anonymous : Write : 15.234 ms > Anonymous : Read+Write : 7.460 ms > Anonymous : POPULATE : 5.196 ms > Anonymous : POPULATE_READ : 5.190 ms > Anonymous : POPULATE_WRITE : 8.245 ms > Memfd 4 KiB : Read : 34.412 ms > Memfd 4 KiB : Write : 30.586 ms > Memfd 4 KiB : Read+Write : 35.157 ms > Memfd 4 KiB : POPULATE : 29.643 ms > Memfd 4 KiB : POPULATE_READ : 29.691 ms > Memfd 4 KiB : POPULATE_WRITE : 25.790 ms > Memfd 2 MiB : Read : 10.210 ms > Memfd 2 MiB : Write : 10.074 ms > Memfd 2 MiB : Read+Write : 10.068 ms > Memfd 2 MiB : POPULATE : 10.034 ms > Memfd 2 MiB : POPULATE_READ : 10.037 ms > Memfd 2 MiB : POPULATE_WRITE : 10.031 ms > ************************************************** > 4096 MiB MAP_PRIVATE: > ************************************************** > Anonymous : Read : 240.947 ms > Anonymous : Write : 712.941 ms > Anonymous : Read+Write : 1027.636 ms > Anonymous : POPULATE : 571.816 ms > Anonymous : POPULATE_READ : 120.215 ms > Anonymous : POPULATE_WRITE : 570.750 ms > Memfd 4 KiB : Read : 1054.739 ms > Memfd 4 KiB : Write : 145.534 ms > Memfd 4 KiB : Read+Write : 202.275 ms > Memfd 4 KiB : POPULATE : 162.597 ms > Memfd 4 KiB : POPULATE_READ : 914.747 ms > Memfd 4 KiB : POPULATE_WRITE : 161.281 ms > Memfd 2 MiB : Read : 351.818 ms > Memfd 2 MiB : Write : 352.357 ms > Memfd 2 MiB : Read+Write : 352.762 ms > Memfd 2 MiB : POPULATE : 351.471 ms > Memfd 2 MiB : POPULATE_READ : 351.553 ms > Memfd 2 MiB : POPULATE_WRITE : 351.931 ms > ************************************************** > 4096 MiB MAP_SHARED: > ************************************************** > Anonymous : Read : 229.338 ms > Anonymous : Write : 478.964 ms > Anonymous : Read+Write : 234.546 ms > Anonymous : POPULATE : 161.635 ms > Anonymous : POPULATE_READ : 160.943 ms > Anonymous : POPULATE_WRITE : 252.686 ms > Memfd 4 KiB : Read : 1052.828 ms > Memfd 4 KiB : Write : 929.237 ms > Memfd 4 KiB : Read+Write : 1074.494 ms > Memfd 4 KiB : POPULATE : 915.663 ms > Memfd 4 KiB : POPULATE_READ : 915.001 ms > Memfd 4 KiB : POPULATE_WRITE : 787.388 ms > Memfd 2 MiB : Read : 353.580 ms > Memfd 2 MiB : Write : 353.197 ms > Memfd 2 MiB : Read+Write : 353.172 ms > Memfd 2 MiB : POPULATE : 353.686 ms > Memfd 2 MiB : POPULATE_READ : 353.465 ms > Memfd 2 MiB : POPULATE_WRITE : 352.776 ms > ************************************************** > > > --- Discussion --- > > 1) With huge pages, the performance benefit is negligible with the sizes > I tried, because there are little actual page faults. Most time is spent > zeroing huge pages I guess. It will take quite a lot of memory to pay off. > > 2) In all 4k cases, the POPULATE_READ/POPULATE_WRITE variants are faster > than manually reading or writing from user space. > > > What sticks out a bit is: > > 3) For MAP_SHARED on anonymous memory, it is fastest to first read and > then write memory. It's slightly faster than POPULATE_WRITE and quite a > lot faster than a simple write - what?!. It's even faster than > POPULATE_WRITE - what?! I assume with the read access we prepare a fresh > zero page and with the write access we only have to change PTE access > rights. But why is this faster than writing directly?
Okay, MADV_DONTNEED does not seem to really work on MAP_SHARED of anonymous memory. If I use a fresh mmap for each and every iteration the numbers make more sense:
************************************************** 4096 MiB MAP_SHARED: ************************************************** Anonymous : Read : 1054.154 ms Anonymous : Write : 924.572 ms Anonymous : Read+Write : 1075.215 ms Anonymous : POPULATE : 911.386 ms Anonymous : POPULATE_READ : 909.392 ms Anonymous : POPULATE_WRITE : 793.143 ms
-- Thanks,
David / dhildenb
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