Re: [PATCH] readahead even for FMODE_RANDOM

[Jens Axboe <jens.axboe@oracle.com>]

On Fri, Apr 02 2010, Wu Fengguang wrote:
> Hi Jens,
> 
> On Fri, Apr 02, 2010 at 02:31:51AM +0800, Jens Axboe wrote:
> > Hi,
> > 
> > I got a problem report with fio where larger block size random reads
> > where markedly slower with buffered IO than with O_DIRECT, and the
> > initial thought was that perhaps this was some fio oddity. The reporter
> > eventually discovered that turning off the fadvise hint made it work
> > fine. So I took a look, and it seems we never do readahead for
> > FMODE_RANDOM even if the request size is larger than 1 page. That seems
> > like a bug, if an application is doing eg 16kb random reads, you want to
> > readahead the 12kb remaining data. On devices where smaller transfer
> > sizes are slower than larger ones, this can make a large difference.
> > 
> > This patch makes us readahead even for FMODE_RANDOM, iff we'll be
> > reading more pages in that single read. I ran a quick test here, and it
> > appears to fix the problem (no difference with fadvise POSIX_FADV_RANDOM
> > being passed in or not).
>  
> I guess the application is doing (at least partial) sequential reads,
> while at the same time tell kernel with POSIX_FADV_RANDOM that it's
> doing random reads.
> 
> If so, it's mainly the application's fault.

The application is doing large random reads. It's purely random, so
the POSIX_FADV_RANDOM hint is correct. However, thinking about it, it
may be that we later hit a random "block" that has now been cached due
to this read-ahead. Let me try and rule that out completely and see if
there's still the difference. The original reporter observed 4kb reads
hitting the driver, where 128kb was expected.

> However the kernel can behave more smart and less "dumb" :)
> It can inherit the current good behavior of "submit one single 16kb io
> request for a 16kb random read() syscall", while still be able to
> start _larger sized_ readahead if it's actually a sequential one.

Yeah, that's an ancient issue and pretty sad.

> > diff --git a/mm/readahead.c b/mm/readahead.c
> > index 337b20e..d4b201c 100644
> > --- a/mm/readahead.c
> > +++ b/mm/readahead.c
> > @@ -501,8 +501,11 @@ void page_cache_sync_readahead(struct address_space *mapping,
> >  	if (!ra->ra_pages)
> >  		return;
> >  
> > -	/* be dumb */
> > -	if (filp->f_mode & FMODE_RANDOM) {
> > +	/*
> > +	 * Be dumb for files marked as randomly accessed, but do readahead
> > +	 * inside the original request (req_size > 1).
> > +	 */
> > +	if ((filp->f_mode & FMODE_RANDOM) && req_size == 1) {
> >  		force_page_cache_readahead(mapping, filp, offset, req_size);
> >  		return;
> >  	}
> 
> The patch only fixes the (req_size != 1) case that exposed by your
> application. A complete fix would be 
> 
> @@ -820,12 +825,6 @@ void page_cache_sync_readahead(struct ad
>  	if (!ra->ra_pages)
>  		return;
>  
> -	/* be dumb */
> -	if (filp->f_mode & FMODE_RANDOM) {
> -		force_page_cache_readahead(mapping, filp, offset, req_size);
> -		return;
> -	}
> -

Hmm, are we talking about the same thing? I want to hit read-ahead for
the remaining pages inside that random read, eg ensure that read-ahead
gets activated inside that window of the random request.

>  	/* do read-ahead */
>  	ondemand_readahead(mapping, ra, filp, false, offset, req_size);
>  }
> 
> And a more optimized patch would look like this.  Note that only the
> last chunk is necessary for bug fixing, and only this chunk can be
> applied to vanilla 2.6.34-rc3.
> 
> If no problem, I'll submit a patch with only the last chunk for
> 2.6.34, and submit the remaining chunks for 2.6.35.
> 
> Thanks,
> Fengguang
> ---
> Subject: readahead: more smart readahead on POSIX_FADV_RANDOM
> From: Wu Fengguang <fengguang.wu@intel.com>
> Date: Fri Apr 02 08:52:42 CST 2010
> 
> Some times user space applications will tell POSIX_FADV_RANDOM
> while still doing some sequential reads.
> 
> The kernel can behave a bit smarter in this case, by letting the
> readahead heuristics handle the POSIX_FADV_RANDOM case, but with
> less aggressive assumption on sequential reads.

I'll try and give this a spin. On the laptop, I cannot reproduce the
problem of smaller < reqsize ios, so hard to say just now.

> 
> CC: Jens Axboe <jens.axboe@oracle.com>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> ---
>  mm/readahead.c |   17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)
> 
> --- linux.orig/mm/readahead.c	2010-04-02 08:43:53.000000000 +0800
> +++ linux/mm/readahead.c	2010-04-02 09:00:51.000000000 +0800
> @@ -664,6 +664,7 @@ ondemand_readahead(struct address_space 
>  	unsigned long max = max_sane_readahead(ra->ra_pages);
>  	unsigned long tt;  /* thrashing shreshold */
>  	pgoff_t start;
> +	bool random_hint = (filp && (filp->f_mode & FMODE_RANDOM));
>  
>  	/*
>  	 * start of file
> @@ -671,7 +672,8 @@ ondemand_readahead(struct address_space 
>  	if (!offset) {
>  		ra_set_pattern(ra, RA_PATTERN_INITIAL);
>  		ra->start = offset;
> -		if ((ra->ra_flags & READAHEAD_LSEEK) && req_size <= max) {
> +		if ((random_hint || (ra->ra_flags & READAHEAD_LSEEK)) &&
> +		    req_size <= max) {
>  			ra->size = req_size;
>  			ra->async_size = 0;
>  			goto readit;
> @@ -743,8 +745,11 @@ context_readahead:
>  	} else
>  		start = offset;
>  
> -	tt = count_history_pages(mapping, ra, offset,
> -				 READAHEAD_ASYNC_RATIO * max);
> +	if (unlikely(random_hint))
> +		tt = 0;
> +	else
> +		tt = count_history_pages(mapping, ra, offset,
> +					 READAHEAD_ASYNC_RATIO * max);
>  	/*
>  	 * no history pages cached, could be
>  	 * 	- a random read
> @@ -820,12 +825,6 @@ void page_cache_sync_readahead(struct ad
>  	if (!ra->ra_pages)
>  		return;
>  
> -	/* be dumb */
> -	if (filp->f_mode & FMODE_RANDOM) {
> -		force_page_cache_readahead(mapping, filp, offset, req_size);
> -		return;
> -	}
> -
>  	/* do read-ahead */
>  	ondemand_readahead(mapping, ra, filp, false, offset, req_size);
>  }

-- 
Jens Axboe