Re: [RFC][PATCH v3] readahead: introduce O_RANDOM for POSIX_FADV_RANDOM

[Minchan Kim]

On Tue, Jan 5, 2010 at 11:16 AM, Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:
> On Tue, Jan 05, 2010 at 09:46:09AM +0800, Minchan Kim wrote:
>> On Mon, Jan 4, 2010 at 9:16 PM, Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:
>> > Hi Minchan,
>> >
>> > On Mon, Jan 04, 2010 at 01:20:49PM +0800, Minchan Kim wrote:
>> >> > --- linux.orig/mm/readahead.c  2010-01-04 12:39:29.000000000 +0800
>> >> > +++ linux/mm/readahead.c    Â2010-01-04 12:39:30.000000000 +0800
>> >> > @@ -501,6 +501,12 @@ void page_cache_sync_readahead(struct ad
>> >> > Â Â Â Âif (!ra->ra_pages)
>> >> > Â Â Â Â Â Â Â Âreturn;
>> >> >
>> >> > + Â Â Â /* be dumb */
>> >> > + Â Â Â if (filp->f_flags & O_RANDOM) {
>> >> > + Â Â Â Â Â Â Â force_page_cache_readahead(mapping, filp, offset, req_size);
>> >> > + Â Â Â Â Â Â Â return;
>> >> > + Â Â Â }
>> >> > +
>> >>
>> >> Let me have a dumb question. :)
>> >>
>> >> How about testing O_RANDOM in front of ra_pages testing?
>> >>
>> >> My intention is that although we turn off ra, it would be better to read
>> >> contiguous block all at once than readpage() callback doing I/O
>> >> one page at a time.
>> >>
>> >> Is it break some semantics or happen some problem in ondemand readahead?
>> >
>> > Yes it will have some problem with shrink_readahead_size_eio(), which
>> > want to disable readahead and use ->readpage() when ra_pages==0.
>> >
>> > Do you have specific use case in mind? The file systems that set
>> > ra_pages=0 seems to don't need readahead, too.
>>
>> Never mind. It's just out of curiosity. :)
>>
>> I thought although user disable readahead, we could enhance file I/O
>> with one readpages not multiple readpage if we know the user want to
>> read big contiguous blocks.
>
> Yes, not-break-large-read-into-pages would be good for HD/SSD drives
> when readahead is disabled.
>
> Currently, ->ra_pages is somehow overloaded in its ==0 case. As you
> said, it's in fact possible to disable readahead while still limiting
> read IO size to a non-zero ->ra_pages.
>
>> But I though it break current readahead off semantics. right?
>
> It can be done by applying the ->ra_pages limit to O_RANDOM. This also
> makes O_RANDOM safer to use:
>
> @@ -497,6 +497,13 @@ void page_cache_sync_readahead(struct ad
> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â struct file_ra_state *ra, struct file *filp,
> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â pgoff_t offset, unsigned long req_size)
> Â{
> + Â Â Â /* be dumb */
> + Â Â Â if (filp->f_flags & O_RANDOM) {
> + Â Â Â Â Â Â Â req_size = clamp_t(unsigned long, req_size, 1, ra->ra_pages);
> + Â Â Â Â Â Â Â force_page_cache_readahead(mapping, filp, offset, req_size);
> + Â Â Â Â Â Â Â return;
> + Â Â Â }
> +
> Â Â Â Â/* no read-ahead */
> Â Â Â Âif (!ra->ra_pages)
> Â Â Â Â Â Â Â Âreturn;
>
> To make real change, we need an interface for the user to disable
> whole-partition readahead by setting O_RANDOM instead of ra_pages=0.
> That would be a hard sell..

Okay. I understand.
Thanks, Wu.

-- 
Kind regards,
Minchan Kim
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