Re: regression in page writeback

Theodore Tso
2009-09-28T14:08:24Z

On Mon, Sep 28, 2009 at 09:08:04AM -0400, Christoph Hellwig wrote:
> This should help a bit for XFS as it historically does multi-page
> writeouts from ->writepages (and apprently btrfs that added some
> write-around recently?) but not those brave filesystems only
> implementing the multi-page writeout from writepages as designed.

Here's the hack which I'm currently working on to work around the
writeback code limiting writebacks to 1024 pages. I'm assuming this
is going to be short-term hack assuming the writeback code gets more
intelligent, but I thought I would throw this into the mix....

- Ted

ext4: Adjust ext4_da_writepages() to write out larger contiguous chunks

Work around problems in the writeback code to force out writebacks in
larger chunks than just 4mb, which is just too small. This also works
around limitations in the ext4 block allocator, which can't allocate
more than 2048 blocks at a time. So we need to defeat the round-robin
characteristics of the writeback code and try to write out as many
blocks in one inode before allowing the writeback code to move on to
another inode. We add a a new per-filesystem tunable,
max_contig_writeback_mb, which caps this to a default of 128mb per
inode.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
---
fs/ext4/ext4.h | 1 +
fs/ext4/inode.c | 100 +++++++++++++++++++++++++++++++++++++-----
fs/ext4/super.c | 3 +
include/trace/events/ext4.h | 14 ++++--
4 files changed, 102 insertions(+), 16 deletions(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index e227eea..9f99427 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -942,6 +942,7 @@ struct ext4_sb_info {
unsigned int s_mb_stats;
unsigned int s_mb_order2_reqs;
unsigned int s_mb_group_prealloc;
+ unsigned int s_max_contig_writeback_mb;
/* where last allocation was done - for stream allocation */
unsigned long s_mb_last_group;
unsigned long s_mb_last_start;
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 5fb72a9..9e0acb7 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1145,6 +1145,60 @@ static int check_block_validity(struct inode *inode, const char *msg,
}

/*
+ * Return the number of dirty pages in the given inode starting at
+ * page frame idx.
+ */
+static pgoff_t ext4_num_dirty_pages(struct inode *inode, pgoff_t idx)
+{
+ struct address_space *mapping = inode->i_mapping;
+ pgoff_t index;
+ struct pagevec pvec;
+ pgoff_t num = 0;
+ int i, nr_pages, done = 0;
+
+ pagevec_init(&pvec, 0);
+
+ while (!done) {
+ index = idx;
+ nr_pages = pagevec_lookup_tag(&pvec, mapping, &index,
+ PAGECACHE_TAG_DIRTY,
+ (pgoff_t)PAGEVEC_SIZE);
+ if (nr_pages == 0)
+ break;
+ for (i = 0; i < nr_pages; i++) {
+ struct page *page = pvec.pages[i];
+ struct buffer_head *bh, *head;
+
+ lock_page(page);
+ if (unlikely(page->mapping != mapping) ||
+ !PageDirty(page) ||
+ PageWriteback(page) ||
+ page->index != idx) {
+ done = 1;
+ unlock_page(page);
+ break;
+ }
+ head = page_buffers(page);
+ bh = head;
+ do {
+ if (!buffer_delay(bh) &&
+ !buffer_unwritten(bh)) {
+ done = 1;
+ break;
+ }
+ } while ((bh = bh->b_this_page) != head);
+ unlock_page(page);
+ if (done)
+ break;
+ idx++;
+ num++;
+ }
+ pagevec_release(&pvec);
+ }
+ return num;
+}
+
+/*
* The ext4_get_blocks() function tries to look up the requested blocks,
* and returns if the blocks are already mapped.
*
@@ -2744,7 +2798,8 @@ static int ext4_da_writepages(struct address_space *mapping,
int pages_written = 0;
long pages_skipped;
int range_cyclic, cycled = 1, io_done = 0;
- int needed_blocks, ret = 0, nr_to_writebump = 0;
+ int needed_blocks, ret = 0;
+ long desired_nr_to_write, nr_to_writebump = 0;
loff_t range_start = wbc->range_start;
struct ext4_sb_info *sbi = EXT4_SB(mapping->host->i_sb);

@@ -2771,16 +2826,6 @@ static int ext4_da_writepages(struct address_space *mapping,
if (unlikely(sbi->s_mount_flags & EXT4_MF_FS_ABORTED))
return -EROFS;

- /*
- * Make sure nr_to_write is >= sbi->s_mb_stream_request
- * This make sure small files blocks are allocated in
- * single attempt. This ensure that small files
- * get less fragmented.
- */
- if (wbc->nr_to_write < sbi->s_mb_stream_request) {
- nr_to_writebump = sbi->s_mb_stream_request - wbc->nr_to_write;
- wbc->nr_to_write = sbi->s_mb_stream_request;
- }
if (wbc->range_start == 0 && wbc->range_end == LLONG_MAX)
range_whole = 1;

@@ -2795,6 +2840,36 @@ static int ext4_da_writepages(struct address_space *mapping,
} else
index = wbc->range_start >> PAGE_CACHE_SHIFT;

+ /*
+ * This works around two forms of stupidity. The first is in
+ * the writeback code, which caps the maximum number of pages
+ * written to be 1024 pages. This is wrong on multiple
+ * levels; different architectues have a different page size,
+ * which changes the maximum amount of data which gets
+ * written. Secondly, 4 megabytes is way too small. XFS
+ * forces this value to be 16 megabytes by multiplying
+ * nr_to_write parameter by four, and then relies on its
+ * allocator to allocate larger extents to make them
+ * contiguous. Unfortunately this brings us to the second
+ * stupidity, which is that ext4's mballoc code only allocates
+ * at most 2048 blocks. So we force contiguous writes up to
+ * the number of dirty blocks in the inode, or
+ * sbi->max_contig_writeback_mb whichever is smaller.
+ */
+ if (!range_cyclic && range_whole)
+ desired_nr_to_write = wbc->nr_to_write * 8;
+ else
+ desired_nr_to_write = ext4_num_dirty_pages(inode, index);
+ if (desired_nr_to_write > (sbi->s_max_contig_writeback_mb <<
+ (20 - PAGE_CACHE_SHIFT)))
+ desired_nr_to_write = (sbi->s_max_contig_writeback_mb <<
+ (20 - PAGE_CACHE_SHIFT));
+
+ if (wbc->nr_to_write < desired_nr_to_write) {
+ nr_to_writebump = desired_nr_to_write - wbc->nr_to_write;
+ wbc->nr_to_write = desired_nr_to_write;
+ }
+
mpd.wbc = wbc;
mpd.inode = mapping->host;

@@ -2914,7 +2989,8 @@ retry:
out_writepages:
if (!no_nrwrite_index_update)
wbc->no_nrwrite_index_update = 0;
- wbc->nr_to_write -= nr_to_writebump;
+ if (wbc->nr_to_write > nr_to_writebump)
+ wbc->nr_to_write -= nr_to_writebump;
wbc->range_start = range_start;
trace_ext4_da_writepages_result(inode, wbc, ret, pages_written);
return ret;
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index df539ba..9d04bd9 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -2197,6 +2197,7 @@ EXT4_RW_ATTR_SBI_UI(mb_min_to_scan, s_mb_min_to_scan);
EXT4_RW_ATTR_SBI_UI(mb_order2_req, s_mb_order2_reqs);
EXT4_RW_ATTR_SBI_UI(mb_stream_req, s_mb_stream_request);
EXT4_RW_ATTR_SBI_UI(mb_group_prealloc, s_mb_group_prealloc);
+EXT4_RW_ATTR_SBI_UI(max_contig_writeback, s_max_contig_writeback_mb);

static struct attribute *ext4_attrs[] = {
ATTR_LIST(delayed_allocation_blocks),
@@ -2210,6 +2211,7 @@ static struct attribute *ext4_attrs[] = {
ATTR_LIST(mb_order2_req),
ATTR_LIST(mb_stream_req),
ATTR_LIST(mb_group_prealloc),
+ ATTR_LIST(max_contig_writeback),
NULL,
};

@@ -2679,6 +2681,7 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
}

sbi->s_stripe = ext4_get_stripe_size(sbi);
+ sbi->s_max_contig_writeback_mb = 128;

/*
* set up enough so that it can read an inode
diff --git a/include/trace/events/ext4.h b/include/trace/events/ext4.h
index c1bd8f1..7c6bbb7 100644
--- a/include/trace/events/ext4.h
+++ b/include/trace/events/ext4.h
@@ -236,6 +236,7 @@ TRACE_EVENT(ext4_da_writepages,
__field( char, for_kupdate )
__field( char, for_reclaim )
__field( char, range_cyclic )
+ __field( pgoff_t, writeback_index )
),

TP_fast_assign(
@@ -249,15 +250,17 @@ TRACE_EVENT(ext4_da_writepages,
__entry->for_kupdate = wbc->for_kupdate;
__entry->for_reclaim = wbc->for_reclaim;
__entry->range_cyclic = wbc->range_cyclic;
+ __entry->writeback_index = inode->i_mapping->writeback_index;
),

- TP_printk("dev %s ino %lu nr_to_write %ld pages_skipped %ld range_start %llu range_end %llu nonblocking %d for_kupdate %d for_reclaim %d range_cyclic %d",
+ TP_printk("dev %s ino %lu nr_to_write %ld pages_skipped %ld range_start %llu range_end %llu nonblocking %d for_kupdate %d for_reclaim %d range_cyclic %d writeback_index %lu",
jbd2_dev_to_name(__entry->dev),
(unsigned long) __entry->ino, __entry->nr_to_write,
__entry->pages_skipped, __entry->range_start,
__entry->range_end, __entry->nonblocking,
__entry->for_kupdate, __entry->for_reclaim,
- __entry->range_cyclic)
+ __entry->range_cyclic,
+ (unsigned long) __entry->writeback_index)
);

TRACE_EVENT(ext4_da_write_pages,
@@ -309,6 +312,7 @@ TRACE_EVENT(ext4_da_writepages_result,
__field( char, encountered_congestion )
__field( char, more_io )
__field( char, no_nrwrite_index_update )
+ __field( pgoff_t, writeback_index )
),

TP_fast_assign(
@@ -320,14 +324,16 @@ TRACE_EVENT(ext4_da_writepages_result,
__entry->encountered_congestion = wbc->encountered_congestion;
__entry->more_io = wbc->more_io;
__entry->no_nrwrite_index_update = wbc->no_nrwrite_index_update;
+ __entry->writeback_index = inode->i_mapping->writeback_index;
),

- TP_printk("dev %s ino %lu ret %d pages_written %d pages_skipped %ld congestion %d more_io %d no_nrwrite_index_update %d",
+ TP_printk("dev %s ino %lu ret %d pages_written %d pages_skipped %ld congestion %d more_io %d no_nrwrite_index_update %d writeback_index %lu",
jbd2_dev_to_name(__entry->dev),
(unsigned long) __entry->ino, __entry->ret,
__entry->pages_written, __entry->pages_skipped,
__entry->encountered_congestion, __entry->more_io,
- __entry->no_nrwrite_index_update)
+ __entry->no_nrwrite_index_update,
+ (unsigned long) __entry->writeback_index)
);

TRACE_EVENT(ext4_da_write_begin,