Hello, I found a weird mmap read behavior while benchmarking the fuse-uring patches. I did not verify yet, but it does not look fuse specific. Basically, I started to check because fio results were much lower than expected (better with the new code, though) fio cmd line: fio --size=1G --numjobs=1 --ioengine=mmap --output-format=normal,terse --directory=/scratch/dest/ --rw=read multi-file.fio bernd@squeeze1 test2>cat multi-file.fio [global] group_reporting bs=1M runtime=300 [test] This sequential fio sets POSIX_MADV_SEQUENTIAL and then does memcpy beginning at offset 0 in 1MB steps (verified with additional logging in fios engines/mmap.c). And additional log in fuse_readahead() gives [ 1396.215084] fuse: 000000003fdec504 inode=00000000be0f29d3 count=64 index=0 [ 1396.237466] fuse: 000000003fdec504 inode=00000000be0f29d3 count=64 index=255 [ 1396.263175] fuse: 000000003fdec504 inode=00000000be0f29d3 count=1 index=254 [ 1396.282055] fuse: 000000003fdec504 inode=00000000be0f29d3 count=1 index=253 ... <count is always 1 page> [ 1496.353745] fuse: 000000003fdec504 inode=00000000be0f29d3 count=1 index=64 [ 1496.381105] fuse: 000000003fdec504 inode=00000000be0f29d3 count=64 index=511 [ 1496.397487] fuse: 000000003fdec504 inode=00000000be0f29d3 count=1 index=510 [ 1496.416385] fuse: 000000003fdec504 inode=00000000be0f29d3 count=1 index=509 ... <count is always 1 page> Logging in do_sync_mmap_readahead() [ 1493.130764] do_sync_mmap_readahead:3015 ino=132 index=0 count=0 ras_start=0 ras_size=0 ras_async=0 ras_ra_pages=64 ras_mmap_miss=0 ras_prev_pos=-1 [ 1493.147173] do_sync_mmap_readahead:3015 ino=132 index=255 count=0 ras_start=0 ras_size=64 ras_async=32 ras_ra_pages=64 ras_mmap_miss=0 ras_prev_pos=-1 [ 1493.165952] do_sync_mmap_readahead:3015 ino=132 index=254 count=0 ras_start=0 ras_size=64 ras_async=32 ras_ra_pages=64 ras_mmap_miss=0 ras_prev_pos=-1 [ 1493.185566] do_sync_mmap_readahead:3015 ino=132 index=253 count=0 ras_start=0 ras_size=64 ras_async=32 ras_ra_pages=64 ras_mmap_miss=0 ras_prev_pos=-1 ... [ 1496.341890] do_sync_mmap_readahead:3015 ino=132 index=64 count=0 ras_start=0 ras_size=64 ras_async=32 ras_ra_pages=64 ras_mmap_miss=0 ras_prev_pos=-1 [ 1496.361385] do_sync_mmap_readahead:3015 ino=132 index=511 count=0 ras_start=96 ras_size=64 ras_async=64 ras_ra_pages=64 ras_mmap_miss=0 ras_prev_pos=-1 So we can see from fuse that it starts to read at page index 0, wants 64 pages (which is actually the double of bdi read_ahead_kb), then skips index 64...254) and immediately goes to index 255. For the mmaped memcpy pages are missing and then it goes back in 1 page steps to get these. A workaround here is to set read_ahead_kb in the bdi to a larger value, another workaround might be (untested) to increase the read-ahead window. Either of these two seem to be workarounds for the index order above. I understand that read-ahead gets limited by the bdi value (although exceeded above), but why does it go back in 1 page steps? My expectation would have been index=0 count=32 (128kb read-head) index=32 count=32 index=64 count=32 ... This is with plain 6.2 + fuse-uring patches. Thanks, Bernd
