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 <[EMAIL PROTECTED]>
> ---
> 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;
>
-
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