Jens Axboe wrote:
On Fri, Jul 08 2005, Andrew Morton wrote:
Jens Axboe <[email protected]> wrote:
Some more investigation - it appears to be broken read-ahead, actually.
hdparm does repeated read(), lseek() loops which causes the read-ahead
logic to mark the file as being in cache (since it reads the same chunk
every time). Killing the INCACHE check (attached) makes it work fine for
me, Grant can you test if it "fixes" it for you as well?
No ideas how to fix the read-ahead logic right now, I pondered some
depedency on sequential but I don't see how it can work correctly for
other cases. Perhaps handle_ra_miss() just isn't being called
appropriately everywhere?
--- mm/readahead.c~ 2005-07-08 11:16:14.000000000 +0200
+++ mm/readahead.c 2005-07-08 11:17:49.000000000 +0200
@@ -351,7 +351,9 @@
ra->cache_hit += nr_to_read;
if (ra->cache_hit >= VM_MAX_CACHE_HIT) {
ra_off(ra);
+#if 0
ra->flags |= RA_FLAG_INCACHE;
+#endif
return 0;
}
} else {
Interesting. We should be turning that back off in handle_ra_miss() as
soon as hdparm seeks away. I'd be suspecting that we're not correctly
undoing the resutls of ra_off() within handle_ra_miss(), except you didn't
comment that bit out.
Or the readahead code is working as intended, and hdparm is doing something
really weird which trips it up.
hdparm should also be misbehaving when run against a regular file, but it
looks like hdparm would need some alterations to test that.
Just use the test app I posted, it shows the problem just fine. If I use
a regular file, behaviour is identical as expected (ie equally broken
:-).
bart:/data1 # ./read_disk ./test 1
Mem Throughput: 101 MiB/sec
Mem Throughput: 103 MiB/sec
Disk Throughput: 22 MiB/sec
bart:/data1 # ./read_disk ./test
Disk Throughput: 29 MiB/sec
I don't see how this is broken. The INCACHE flag should be cleared on
the first miss. We will start up I/O slightly slower in this case since
it is not 1st IO, but within about 5 or 6 reads we should be back to
normal max_readahead behavior. This is what I see on my old laptop.
patches:/home/slpratt # ./readdisk /dev/hda
Disk Throughput: 16 MiB/sec
patches:/home/slpratt # ./readdisk /dev/hda 1
Mem Throughput: 145 MiB/sec
Mem Throughput: 153 MiB/sec
Disk Throughput: 16 MiB/sec
Can you post iostat -x output from your runs. Are we doing the right
sized IO?
Steve
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