On Thu 02-08-18 12:58:04, Theodore Y. Ts'o wrote: > On Thu, Aug 02, 2018 at 01:56:41PM +0000, Bean Huo (beanhuo) wrote: > > > > I am newbie on ext4, I tried the above method to disable readahead, > > echo 0 > /sys/block/<dev>/queue/read_ahead_kb Then I read by 128kB > > chunk size, ext4 will read the file by 4KB chunk size each > > time. that means ext4 splits 128KB into 32 4KB to read. That's not > > my expectation. Do you know how to still keep and let ext4 read by > > 128KB in case of disable readahead? > > Hmm... that's not my expectation as well, but I've replicated your > results. More interestingly, I tried the same experiment using XFS, > and it does the same thing. I used as my test workload: > > dd if=/mnt/test bs=128k count=32 | sum > > Used strace to verify that dd was in fact issuing 128k reads: > > read(0, "\377\253a)\307\10\230\6\360,,:\226Rq\204\343\2522&44\307\341\372\271\271/\224#?\346"..., 131072) = 131072 > write(1, "\377\253a)\307\10\230\6\360,,:\226Rq\204\343\2522&44\307\341\372\271\271/\224#?\346"..., 131072) = 131072 > > And then used btrace to monitor the I/O requests sent to the device: > > 252,4 0 413 0.077274997 14645 Q R 4408 + 8 [dd] > 252,4 2 77 0.077355648 5529 C R 4408 + 8 [0] > 252,4 0 414 0.077393725 14645 Q R 4416 + 8 [dd] > 252,4 2 78 0.077630722 5529 C R 4416 + 8 [0] > ... > > ... and indeed, the reads are being sent to the device in 4k chunks. > That's indeed surprising. I'd have to do some debugging with > tracepoints to see what requests are being issued from the > mm/filemap.c to the file system. And this is in fact expected. There are two basic ways how data can appear in page cache: ->readpage and ->readpages filesystem callbacks. The second one is what readahead (and only readahead) uses, the first one is used as a fallback when readahead fails for some reason. So if you disable readahead, you're left only with ->readpage call which does only one-page (4k) reads. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
