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 2010-01-04 12:39:30.000000000 +0800
> >> > @@ -501,6 +501,12 @@ void page_cache_sync_readahead(struct ad
> >> > if (!ra->ra_pages)
> >> > return;
> >> >
> >> > + /* be dumb */
> >> > + if (filp->f_flags & O_RANDOM) {
> >> > + force_page_cache_readahead(mapping, filp, offset, req_size);
> >> > + return;
> >> > + }
> >> > +
> >>
> >> 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
struct file_ra_state *ra, struct file *filp,
pgoff_t offset, unsigned long req_size)
{
+ /* be dumb */
+ if (filp->f_flags & O_RANDOM) {
+ req_size = clamp_t(unsigned long, req_size, 1, ra->ra_pages);
+ force_page_cache_readahead(mapping, filp, offset, req_size);
+ return;
+ }
+
/* no read-ahead */
if (!ra->ra_pages)
return;
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..
> Thanks for reply about my dumb question, Wu. :)
You are welcome :)
Thanks,
Fengguang
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