From: Ni zhan Chen <nizhan.chen@gmail.com>
To: Dave Chinner <david@fromorbit.com>
Cc: Fengguang Wu <fengguang.wu@intel.com>,
YingHang Zhu <casualfisher@gmail.com>,
akpm@linux-foundation.org, linux-fsdevel@vger.kernel.org,
linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH] mm: readahead: remove redundant ra_pages in file_ra_state
Date: Fri, 26 Oct 2012 10:25:02 +0800 [thread overview]
Message-ID: <5089F47E.5010405@gmail.com> (raw)
In-Reply-To: <20121026002544.GI29378@dastard>
On 10/26/2012 08:25 AM, Dave Chinner wrote:
> On Thu, Oct 25, 2012 at 10:58:26AM +0800, Fengguang Wu wrote:
>> Hi Chen,
>>
>>> But how can bdi related ra_pages reflect different files' readahead
>>> window? Maybe these different files are sequential read, random read
>>> and so on.
>> It's simple: sequential reads will get ra_pages readahead size while
>> random reads will not get readahead at all.
>>
>> Talking about the below chunk, it might hurt someone that explicitly
>> takes advantage of the behavior, however the ra_pages*2 seems more
>> like a hack than general solution to me: if the user will need
>> POSIX_FADV_SEQUENTIAL to double the max readahead window size for
>> improving IO performance, then why not just increase bdi->ra_pages and
>> benefit all reads? One may argue that it offers some differential
>> behavior to specific applications, however it may also present as a
>> counter-optimization: if the root already tuned bdi->ra_pages to the
>> optimal size, the doubled readahead size will only cost more memory
>> and perhaps IO latency.
>>
>> --- a/mm/fadvise.c
>> +++ b/mm/fadvise.c
>> @@ -87,7 +86,6 @@ SYSCALL_DEFINE(fadvise64_64)(int fd, loff_t offset, loff_t len, int advice)
>> spin_unlock(&file->f_lock);
>> break;
>> case POSIX_FADV_SEQUENTIAL:
>> - file->f_ra.ra_pages = bdi->ra_pages * 2;
> I think we really have to reset file->f_ra.ra_pages here as it is
> not a set-and-forget value. e.g. shrink_readahead_size_eio() can
> reduce ra_pages as a result of IO errors. Hence if you have had io
> errors, telling the kernel that you are now going to do sequential
> IO should reset the readahead to the maximum ra_pages value
> supported....
Good catch!
>
> Cheers,
>
> Dave.
next prev parent reply other threads:[~2012-10-26 2:25 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-10-23 12:46 [PATCH] mm: readahead: remove redundant ra_pages in file_ra_state Ying Zhu
2012-10-23 13:21 ` Ni zhan Chen
[not found] ` <CAA9v8mGMa3SDD1OLTG_wdhCGx7K-0kvSV1+MRi9uCGTz6zZaLg@mail.gmail.com>
2012-10-23 13:41 ` YingHang Zhu
2012-10-24 1:02 ` Ni zhan Chen
2012-10-24 1:33 ` YingHang Zhu
2012-10-23 22:47 ` Dave Chinner
2012-10-23 23:53 ` YingHang Zhu
2012-10-24 20:19 ` Dave Chinner
2012-10-25 0:17 ` YingHang Zhu
2012-10-25 1:48 ` Ni zhan Chen
2012-10-25 1:50 ` Dave Chinner
2012-10-25 2:04 ` YingHang Zhu
2012-10-25 2:12 ` Ni zhan Chen
2012-10-25 2:31 ` YingHang Zhu
2012-10-25 2:58 ` Fengguang Wu
2012-10-25 3:12 ` YingHang Zhu
2012-10-26 0:25 ` Dave Chinner
2012-10-26 1:27 ` Fengguang Wu
2012-10-26 2:30 ` Ni zhan Chen
2012-10-26 3:28 ` YingHang Zhu
2012-10-26 3:51 ` Ni zhan Chen
2012-10-26 4:35 ` YingHang Zhu
2012-10-26 6:58 ` Fengguang Wu
2012-10-26 7:03 ` Ni zhan Chen
2012-10-26 7:09 ` Fengguang Wu
2012-10-26 7:19 ` Ni zhan Chen
2012-10-26 7:36 ` Fengguang Wu
2012-10-26 7:47 ` Ni zhan Chen
2012-10-26 8:02 ` Fengguang Wu
2012-10-26 8:08 ` Ni zhan Chen
2012-10-26 8:13 ` YingHang Zhu
2012-10-26 2:25 ` Ni zhan Chen [this message]
2012-10-26 3:38 ` YingHang Zhu
2012-10-26 3:55 ` Fengguang Wu
2012-10-26 5:00 ` YingHang Zhu
2012-10-25 2:38 ` Fengguang Wu
2012-10-25 3:08 ` YingHang Zhu
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