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Subject[PATCH] VFS: Pagecache usage optimization on pagesize != blocksize environment
Hi.

When we read some part of a file through pagecache, if there is a pagecache
of corresponding index but this page is not uptodate, read IO is issued and
this page will be uptodate.
I think this is good for pagesize == blocksize environment but there is room
for improvement on pagesize != blocksize environment. Because in this case
a page can have multiple buffers and even if a page is not uptodate, some buffers
can be uptodate. So I suggest that when all buffers which correspond to a part
of a file that we want to read are uptodate, use this pagecache and copy data
from this pagecache to user buffer even if a page is not uptodate. This can
reduce read IO and improve system throughput.

I did a performance test using the sysbench.

#sysbench --num-threads=4 --max-requests=120000 --test=fileio --file-num=1 --file-block-size=1K --file-total-size=100M --file-test-mode=rndrw --file-fsync-freq=0 --file-rw-ratio=0.5 run
The result was:

-- 2.6.26-rc3
Operations performed: 40002 Read, 79998 Write, 1 Other = 120001 Total
Read 39.064Mb Written 78.123Mb Total transferred 117.19Mb (375Kb/sec)
375.00 Requests/sec executed

Test execution summary:
total time: 320.0027s
total number of events: 120000
total time taken by event execution: 1231.5564
per-request statistics:
min: 0.0000s
avg: 0.0103s
max: 2.7605s
approx. 95 percentile: 0.0381s

-- 2.6.26-rc3-patched
Operations performed: 40002 Read, 79998 Write, 1 Other = 120001 Total
Read 39.064Mb Written 78.123Mb Total transferred 117.19Mb (409.78Kb/sec)
409.78 Requests/sec executed

Test execution summary:
total time: 292.8406s
total number of events: 120000
total time taken by event execution: 1106.3995
per-request statistics:
min: 0.0000s
avg: 0.0092s
max: 3.7366s
approx. 95 percentile: 0.0327s

arch:i386
filesystem:ext3
blocksize:1024 bytes
Memory: 1GB

Random read/write throughput was somewhat improved with following patch.
Thanks.

Signed-off-by :Hisashi Hifumi <hifumi.hisashi@oss.ntt.co.jp>

diff -Nrup linux-2.6.26-rc3.org/fs/buffer.c linux-2.6.26-rc3/fs/buffer.c
--- linux-2.6.26-rc3.org/fs/buffer.c 2008-05-19 11:35:10.000000000 +0900
+++ linux-2.6.26-rc3/fs/buffer.c 2008-05-19 14:29:25.000000000 +0900
@@ -2084,6 +2084,48 @@ int generic_write_end(struct file *file,
EXPORT_SYMBOL(generic_write_end);

/*
+ * check_buffers_uptodate checks whether buffers within a page are
+ * uptodate or not.
+ *
+ * Returns true if all buffers which correspond to a file portion
+ * we want to read are uptodate.
+ */
+int check_buffers_uptodate(unsigned long from,
+ read_descriptor_t *desc, struct page *page)
+{
+ struct inode *inode = page->mapping->host;
+ unsigned long block_start, block_end, blocksize;
+ unsigned long to;
+ struct buffer_head *bh, *head;
+ int ret = 1;
+
+ blocksize = 1 << inode->i_blkbits;
+ to = from + desc->count;
+ if (to > PAGE_CACHE_SIZE)
+ to = PAGE_CACHE_SIZE;
+ if (from < blocksize && to > PAGE_CACHE_SIZE - blocksize)
+ return 0;
+
+ head = page_buffers(page);
+
+ for (bh = head, block_start = 0; bh != head || !block_start;
+ block_start = block_end, bh = bh->b_this_page) {
+ block_end = block_start + blocksize;
+ if (block_end <= from || block_start >= to)
+ continue;
+ else {
+ if (!buffer_uptodate(bh)) {
+ ret = 0;
+ break;
+ }
+ if (block_end >= to)
+ break;
+ }
+ }
+ return ret;
+}
+
+/*
* Generic "read page" function for block devices that have the normal
* get_block functionality. This is most of the block device filesystems.
* Reads the page asynchronously --- the unlock_buffer() and
diff -Nrup linux-2.6.26-rc3.org/include/linux/buffer_head.h linux-2.6.26-rc3/include/linux/buffer_head.h
--- linux-2.6.26-rc3.org/include/linux/buffer_head.h 2008-05-19 11:35:11.000000000 +0900
+++ linux-2.6.26-rc3/include/linux/buffer_head.h 2008-05-19 12:13:46.000000000 +0900
@@ -205,6 +205,8 @@ void block_invalidatepage(struct page *p
int block_write_full_page(struct page *page, get_block_t *get_block,
struct writeback_control *wbc);
int block_read_full_page(struct page*, get_block_t*);
+int check_buffers_uptodate(unsigned long from,
+ read_descriptor_t *desc, struct page *page);
int block_write_begin(struct file *, struct address_space *,
loff_t, unsigned, unsigned,
struct page **, void **, get_block_t*);
diff -Nrup linux-2.6.26-rc3.org/mm/filemap.c linux-2.6.26-rc3/mm/filemap.c
--- linux-2.6.26-rc3.org/mm/filemap.c 2008-05-19 11:35:11.000000000 +0900
+++ linux-2.6.26-rc3/mm/filemap.c 2008-05-19 14:29:23.000000000 +0900
@@ -932,8 +932,16 @@ find_page:
ra, filp, page,
index, last_index - index);
}
- if (!PageUptodate(page))
- goto page_not_up_to_date;
+ if (!PageUptodate(page)) {
+ if (inode->i_blkbits == PAGE_CACHE_SHIFT)
+ goto page_not_up_to_date;
+ if (TestSetPageLocked(page))
+ goto page_not_up_to_date;
+ if (!page_has_buffers(page) ||
+ !check_buffers_uptodate(offset, desc, page))
+ goto page_not_up_to_date_locked;
+ unlock_page(page);
+ }
page_ok:
/*
* i_size must be checked after we know the page is Uptodate.
@@ -1003,6 +1011,7 @@ page_not_up_to_date:
if (lock_page_killable(page))
goto readpage_eio;

+page_not_up_to_date_locked:
/* Did it get truncated before we got the lock? */
if (!page->mapping) {
unlock_page(page);


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