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Date: Fri, 2 Apr 2010 09:23:38 +0800
From: Wu Fengguang <fengguang.wu@...el.com>
To: Jens Axboe <jens.axboe@...cle.com>
Cc: Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] readahead even for FMODE_RANDOM
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@...cle.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@...el.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@...cle.com>
Signed-off-by: Wu Fengguang <fengguang.wu@...el.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);
}
--
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