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 <[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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