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
