Re: [PATCH] readahead even for FMODE_RANDOM

[Wu Fengguang <fengguang.wu@intel.com>]

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.

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.

> Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
> 
> 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;
-	}
-
 	/* 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.

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);
 }