disk throughput

Andrew Morton
2001-11-05T02:18:49Z

I've been taking a look at one particular workload - the creation
and use of many smallish files. ie: the things we do every day
with kernel trees.

There are a number of things which cause linux to perform quite badly
with this workload. I've fixed most of them and the speedups are quite
dramatic. The changes include:

- reorganise the sync_inode() paths so we don't do
read-a-block,write-a-block,read-a-block, ...

- asynchronous preread of an ext2 inode's block when we
create the inode, so:

a) the reads will cluster into larger requests and
b) during inode writeout we don't keep stalling on
reads, preventing write clustering.

- Move ext2's bitmap loading code outside lock_super(),
so other threads can still get in and write to the
fs while the reader sleeps, thus increasing write
request merging. This benefits multithreaded workloads
(ie: dbench) and needs more thought.

The above changes are basically a search-and-destroy mission against
the things which are preventing effective writeout request merging.
Once they're in place we also need:

- Alter the request queue size and elvtune settings

The time to create 100,000 4k files (10 per directory) has fallen
from 3:09 (3min 9second) down to 0:30. A six-fold speedup.

All well and good, and still a WIP. But by far the most dramatic
speedups come from disabling ext2's policy of placing new directories
in a different blockgroup from the parent:

--- linux-2.4.14-pre8/fs/ext2/ialloc.c Tue Oct 9 21:31:40 2001
+++ linux-akpm/fs/ext2/ialloc.c Sun Nov 4 17:40:43 2001
@@ -286,7 +286,7 @@ struct inode * ext2_new_inode (const str
repeat:
gdp = NULL; i=0;

- if (S_ISDIR(mode)) {
+ if (0 && S_ISDIR(mode)) {
avefreei = le32_to_cpu(es->s_free_inodes_count) /
sb->u.ext2_sb.s_groups_count;
/* I am not yet convinced that this next bit is necessary.

Numbers. The machine has 768 megs; the disk is IDE with a two meg cache.
The workload consists of untarring, tarring, diffing and removing kernel
trees. This filesystem is 21 gigs, and has 176 block groups.

After each test which wrote data a `sync' was performed, and was included
in the timing under the assumption that all the data will be written back
by kupdate in a few seconds, and running `sync' allows measurement of the
cost of that.

The filesystem was unmounted between each test - all tests are with
cold caches.

stock patched
untar one kernel tree, sync: 0:31 0:14
diff two trees: 3:04 1:12
tar kernel tree to /dev/null: 0:15 0:03
remove 2 kernel trees, sync: 0:30 0:10

A significant thing here is the improvement in read performance as well
as writes. All of the other speedup changes only affect writes.

We are paying an extremely heavy price for placing directories in
different block groups from their parent. Why do we do this, and
is it worth the cost?

-