linux-ext4 - RE: [EXT] how to disable readahead

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Message-ID: <c642c51dc48e45299c7e19d57e8d3770@SIWEX5A.sing.micron.com>
Date:   Sat, 15 Sep 2018 12:42:15 +0000
From:   "Bean Huo (beanhuo)" <beanhuo@...ron.com>
To:     Jan Kara <jack@...e.cz>, Andreas Dilger <adilger@...ger.ca>,
        "jeffm@...e.com" <jeffm@...e.com>
CC:     "Theodore Y. Ts'o" <tytso@....edu>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: RE: [EXT] how to disable readahead

>Subject: Re: [EXT] how to disable readahead
>
>On Wed 12-09-18 12:29:50, Andreas Dilger wrote:
>> On Sep 12, 2018, at 9:13 AM, Jan Kara <jack@...e.cz> 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\2
>> >> 71\271/\224#?\346"..., 131072) = 131072 write(1,
>> >>
>"\377\253a)\307\10\230\6\360,,:\226Rq\204\343\2522&44\307\341\372\2
>> >> 71\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()?
>
>Hum, I don't understand. With readahead enabled, we should be submitting
>larger batches of IO as generated by ->readpages call and ->readpage actually
>never ends up issuing any IO (see how generic_file_buffered_read() calls
>page_cache_sync_readahead() first which ends up locking pages and
>submitting reads) and only then we go, search for the page again and lock it -
>which effectively waits for the readahead to pull in the first page.
>
>								Honza
>--
>Jan Kara <jack@...e.com>
>SUSE Labs, CR

'read_ahead_kb' should be only used for the read ahead (second time read internal),
should be used as a flag to change the first read request chunk size came from user space read.
Even the 'read_ahead_kb' configured 0.

-Beanhuo