The radix-tree walk for writeback has a couple of problems:
a) It always scans a file from its first dirty page, so if someone
is repeatedly dirtying the front part of a file, pages near the end
may be starved of writeout. (Well, not completely: the `kupdate'
function will write an entire file once the file's dirty timestamp
has expired).
b) When the disk queues are huge (10000 requests), there can be a
very large number of locked pages. Scanning past these in writeback
consumes quite some CPU time.
So in each address_space we record the index at which the last batch of
writeout terminated and start the next batch of writeback from that
point.
---
25-akpm/fs/mpage.c | 20 +++++++++++++++++++-
25-akpm/include/linux/fs.h | 1 +
2 files changed, 20 insertions(+), 1 deletion(-)
diff -puN fs/mpage.c~writeback-search-start fs/mpage.c
--- 25/fs/mpage.c~writeback-search-start 2004-03-23 23:29:02.868978784 -0800
+++ 25-akpm/fs/mpage.c 2004-03-23 23:30:28.099021848 -0800
@@ -610,6 +610,7 @@ mpage_writepages(struct address_space *m
struct pagevec pvec;
int nr_pages;
pgoff_t index;
+ int scanned = 0;
if (wbc->nonblocking && bdi_write_congested(bdi)) {
wbc->encountered_congestion = 1;
@@ -621,11 +622,18 @@ mpage_writepages(struct address_space *m
writepage = mapping->a_ops->writepage;
pagevec_init(&pvec, 0);
- index = 0;
+ if (wbc->sync_mode == WB_SYNC_NONE) {
+ index = mapping->writeback_index; /* Start from prev offset */
+ } else {
+ index = 0; /* whole-file sweep */
+ scanned = 1;
+ }
+retry:
while (!done && (nr_pages = pagevec_lookup_tag(&pvec, mapping, &index,
PAGECACHE_TAG_DIRTY, PAGEVEC_SIZE))) {
unsigned i;
+ scanned = 1;
for (i = 0; i < nr_pages; i++) {
struct page *page = pvec.pages[i];
@@ -672,6 +680,16 @@ mpage_writepages(struct address_space *m
}
pagevec_release(&pvec);
}
+ if (!scanned && !done) {
+ /*
+ * We hit the last page and there is more work to be done: wrap
+ * back to the start of the file
+ */
+ scanned = 1;
+ index = 0;
+ goto retry;
+ }
+ mapping->writeback_index = index;
if (bio)
mpage_bio_submit(WRITE, bio);
return ret;
diff -puN include/linux/fs.h~writeback-search-start include/linux/fs.h
--- 25/include/linux/fs.h~writeback-search-start 2004-03-23 23:29:02.870978480 -0800
+++ 25-akpm/include/linux/fs.h 2004-03-23 23:29:02.873978024 -0800
@@ -327,6 +327,7 @@ struct address_space {
struct radix_tree_root page_tree; /* radix tree of all pages */
spinlock_t tree_lock; /* and spinlock protecting it */
unsigned long nrpages; /* number of total pages */
+ pgoff_t writeback_index;/* writeback starts here */
struct address_space_operations *a_ops; /* methods */
struct list_head i_mmap; /* list of private mappings */
struct list_head i_mmap_shared; /* list of shared mappings */
_