Re: resend--[PATCH] improve read ahead in kernel

[Roman Gushchin <klamm@yandex-team.ru>]

Hi Simon,

20.12.2012, 10:21, "Simon Jeons" <simon.jeons@gmail.com>:
> On Sun, 2012-12-16 at 02:15 +0000, Eric Wong wrote:
>
>> ?xtu4 <xiaobing.tu@intel.com> wrote:
>>> ?resend it, due to format error
>>>
>>> ?Subject: [PATCH] when system in low memory scenario, imaging there is a mp3
>>> ??play, ora video play, we need to read mp3 or video file
>>> ??from memory to page cache,but when system lack of memory,
>>> ??page cache of mp3 or video file will be reclaimed.once read
>>> ??in memory, then reclaimed, it will cause audio or video
>>> ??glitch,and it will increase the io operation at the same
>>> ??time.
>> ?To me, this basically describes how POSIX_FADV_NOREUSE should work.
>
> Hi Eric,
>
> But why fadvise POSIX_FADV_NOREUSE almost do nothing? Why not set some
> flag or other things for these use once data?

Because, it's not clear how should it work in some cases.
Do we expect one access? Should we track a page as accessed after any access (even if only one byte was read)?
What should we do with already-cached pages? etc.

IMHO, it will be better to introduce something like POSIX_FADV_DONTCACHE.
Corresponding pages can be added (ratated) to the tail of the inactive LRU after copying data in read() internals,
or after minor/major pagefault.

>
>> ?I would like to have this ability via fadvise (and not CONFIG_).
>>
>> ?Also, I think your patch has too many #ifdefs to be accepted.
>>
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