* [PATCH -mm] readahead: partial sendfile fix
[not found] <20070210014049.GA11269@mail.ustc.edu.cn>
@ 2007-02-10 1:40 ` Fengguang Wu
2007-02-12 19:49 ` Ram Pai
1 sibling, 0 replies; 3+ messages in thread
From: Fengguang Wu @ 2007-02-10 1:40 UTC (permalink / raw)
To: Andrew Morton
Cc: Suparna Bhattacharya, John Tran, Mike Sullivan, Chris Mason,
Steven Pratt, Ram Pai, Badari Pulavarty, Mingming Cao,
Linux Kernel
Enable readahead to handle partially done read requests, e.g.
sendfile(188, 1921, [1478592], 19553028) = 37440
sendfile(188, 1921, [1516032], 19515588) = 28800
sendfile(188, 1921, [1544832], 19486788) = 37440
sendfile(188, 1921, [1582272], 19449348) = 14400
sendfile(188, 1921, [1596672], 19434948) = 37440
sendfile(188, 1921, [1634112], 19397508) = 37440
In the above strace log,
- some lighttpd is doing _sequential_ reading
- every sendfile() returns with only _partial_ work done
page_cache_readahead() expects that if it returns @next_index, it will be
called exactly at @next_index next time. That's not true here. So the pattern
will be falsely recognized as a random read trace.
Also documented in "Linux AIO Performance and Robustness for Enterprise
Workloads" section 3.5:
sendfile(fd, 0, 2GB, fd2) = 8192,
tells readahead about up to 128KB of the read
sendfile(fd, 8192, 2GB - 8192, fd2) = 8192,
tells readahead about 8KB - 132KB of the read
sendfile(fd, 16384, 2GB - 16384, fd2) = 8192,
tells readahead about 16KB-140KB of the read
...
This confuses the readahead logic about the I/O pattern which appears
to be 0-128K, 8K-132K, 16K-140K instead of clear sequentiality from
0-2GB that is really appropriate.
Retry based AIO shares the same read pattern and readahead problem.
In this case, simply disabling readahead on restarted aio is not a good option:
we still need to call into readahead in the rare case of (req_size > ra_max).
Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
---
mm/filemap.c | 3 ---
mm/readahead.c | 9 +++++++++
2 files changed, 9 insertions(+), 3 deletions(-)
--- linux-2.6.20-rc6-mm3.orig/mm/readahead.c
+++ linux-2.6.20-rc6-mm3/mm/readahead.c
@@ -581,6 +581,15 @@ page_cache_readahead(struct address_spac
int sequential;
/*
+ * A previous read request is partially completed,
+ * causing the retried/continued read calls into us prematurely.
+ */
+ if (ra->start < offset &&
+ offset < ra->prev_page &&
+ ra->prev_page < ra->ahead_start + ra->ahead_size)
+ goto out;
+
+ /*
* We avoid doing extra work and bogusly perturbing the readahead
* window expansion logic.
*/
--- linux-2.6.20-rc6-mm3.orig/mm/filemap.c
+++ linux-2.6.20-rc6-mm3/mm/filemap.c
@@ -915,9 +915,6 @@ void do_generic_mapping_read(struct addr
if (!isize)
goto out;
- if (unlikely(aio_restarted()))
- next_index = last_index; /* Avoid repeat readahead */
-
end_index = (isize - 1) >> PAGE_CACHE_SHIFT;
for (;;) {
struct page *page;
^ permalink raw reply [flat|nested] 3+ messages in thread* Re: [PATCH -mm] readahead: partial sendfile fix
[not found] <20070210014049.GA11269@mail.ustc.edu.cn>
2007-02-10 1:40 ` [PATCH -mm] readahead: partial sendfile fix Fengguang Wu
@ 2007-02-12 19:49 ` Ram Pai
[not found] ` <20070308102549.GA5908@mail.ustc.edu.cn>
1 sibling, 1 reply; 3+ messages in thread
From: Ram Pai @ 2007-02-12 19:49 UTC (permalink / raw)
To: Fengguang Wu
Cc: Andrew Morton, Suparna Bhattacharya, John Tran, Mike Sullivan,
Chris Mason, Steven Pratt, Badari Pulavarty, Mingming Cao,
Linux Kernel
On Sat, 2007-02-10 at 09:40 +0800, Fengguang Wu wrote:
> Enable readahead to handle partially done read requests, e.g.
>
> sendfile(188, 1921, [1478592], 19553028) = 37440
> sendfile(188, 1921, [1516032], 19515588) = 28800
> sendfile(188, 1921, [1544832], 19486788) = 37440
> sendfile(188, 1921, [1582272], 19449348) = 14400
> sendfile(188, 1921, [1596672], 19434948) = 37440
> sendfile(188, 1921, [1634112], 19397508) = 37440
>
> In the above strace log,
> - some lighttpd is doing _sequential_ reading
> - every sendfile() returns with only _partial_ work done
>
> page_cache_readahead() expects that if it returns @next_index, it will
> be
> called exactly at @next_index next time. That's not true here. So the
> pattern
> will be falsely recognized as a random read trace.
>
> Also documented in "Linux AIO Performance and Robustness for
> Enterprise
> Workloads" section 3.5:
>
> sendfile(fd, 0, 2GB, fd2) = 8192,
> tells readahead about up to 128KB of the read
> sendfile(fd, 8192, 2GB - 8192, fd2) = 8192,
> tells readahead about 8KB - 132KB of the read
> sendfile(fd, 16384, 2GB - 16384, fd2) = 8192,
> tells readahead about 16KB-140KB of the read
> ...
> This confuses the readahead logic about the I/O pattern which
> appears
> to be 0-128K, 8K-132K, 16K-140K instead of clear sequentiality
> from
> 0-2GB that is really appropriate.
>
> Retry based AIO shares the same read pattern and readahead problem.
> In this case, simply disabling readahead on restarted aio is not a
> good option:
> we still need to call into readahead in the rare case of (req_size >
> ra_max).
The solution you proposed seems kludgy to me. If you determine that the
its a restarted aio, then start reading from where readahead had left
reading from earlier. To me a simple fix is:
- if (unlikely(aio_restarted()))
- next_index = last_index; /* Avoid repeat readahead */
+ if (unlikely(aio_restarted()))
+ next_index = min(prev_index+1, last_index);
No?
RP
>
> Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
> ---
> mm/filemap.c | 3 ---
> mm/readahead.c | 9 +++++++++
> 2 files changed, 9 insertions(+), 3 deletions(-)
>
> --- linux-2.6.20-rc6-mm3.orig/mm/readahead.c
> +++ linux-2.6.20-rc6-mm3/mm/readahead.c
> @@ -581,6 +581,15 @@ page_cache_readahead(struct address_spac
> int sequential;
>
> /*
> + * A previous read request is partially completed,
> + * causing the retried/continued read calls into us
> prematurely.
> + */
> + if (ra->start < offset &&
> + offset < ra->prev_page &&
> + ra->prev_page < ra->ahead_start +
> ra->ahead_size)
> + goto out;
> +
> + /*
> * We avoid doing extra work and bogusly perturbing the
> readahead
> * window expansion logic.
> */
> --- linux-2.6.20-rc6-mm3.orig/mm/filemap.c
> +++ linux-2.6.20-rc6-mm3/mm/filemap.c
> @@ -915,9 +915,6 @@ void do_generic_mapping_read(struct addr
> if (!isize)
> goto out;
>
> - if (unlikely(aio_restarted()))
> - next_index = last_index; /* Avoid repeat readahead */
> -
> end_index = (isize - 1) >> PAGE_CACHE_SHIFT;
> for (;;) {
> struct page *page;
>
^ permalink raw reply [flat|nested] 3+ messages in thread