On Sep 12, 2018, at 9:13 AM, Jan Kara <jack@xxxxxxx> wrote: > > 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. Even *with* readahead, why would we add the overhead of processing each page separately instead of handling all pages in a single batch via readpages()? Cheers, Andreas
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