Re: Understanding I/O behaviour - next try

[Jens Axboe]

On Tue, Aug 28 2007, Martin Knoblauch wrote:
> Keywords: I/O, bdi-v9, cfs
> 
> Hi,
> 
>  a while ago I asked a few questions on the Linux I/O behaviour,
> because I were (still am) fighting some "misbehaviour" related to heavy
> I/O.
> 
>  The basic setup is a dual x86_64 box with 8 GB of memory. The DL380
> has a HW RAID5, made from 4x72GB disks and about 100 MB write cache.
> The performance of the block device with O_DIRECT is about 90 MB/sec.
> 
>  The problematic behaviour comes when we are moving large files through
> the system. The file usage in this case is mostly "use once" or
> streaming. As soon as the amount of file data is larger than 7.5 GB, we
> see occasional unresponsiveness of the system (e.g. no more ssh
> connections into the box) of more than 1 or 2 minutes (!) duration
> (kernels up to 2.6.19). Load goes up, mainly due to pdflush threads and
> some other poor guys being in "D" state.
> 
>  The data flows in basically three modes. All of them are affected:
> 
> local-disk -> NFS
> NFS -> local-disk
> NFS -> NFS
> 
>  NFS is V3/TCP.
> 
>  So, I made a few experiments in the last few days, using three
> different kernels: 2.6.22.5, 2.6.22.5+cfs20.4 an 2.6.22.5+bdi-v9.
> 
>  The first observation (independent of the kernel) is that we *should*
> use O_DIRECT, at least for output to the local disk. Here we see about
> 90 MB/sec write performance. A simple "dd" using 1,2 and 3 parallel
> threads to the same block device (through a ext2 FS) gives:
> 
> O_Direct: 88 MB/s, 2x44, 3x29.5
> non-O_DIRECT: 51 MB/s, 2x19, 3x12.5
> 
> - Observation 1a: IO schedulers are mostly equivalent, with CFQ
> slightly worse than AS and DEADLINE
> - Observation 1b: when using a 2.6.22.5+cfs20.4, the non-O_DIRECT
> performance goes [slightly] down. With three threads it is 3x10 MB/s.
> Ingo?
> - Observation 1c: bdi-v9 does not help in this case, which is not
> surprising.
> 
>  The real question here is why the non-O_DIRECT case is so slow. Is
> this a general thing? Is this related to the CCISS controller? Using
> O_DIRECT is unfortunatelly not an option for us.
> 
>  When using three different targets (local disk plus two different NFS
> Filesystems) bdi-v9 is a big winner. Without it, all threads are [seem
> to be] limited to the speed of the slowest FS. With bdi-v9 we see a
> considerable speedup.
> 
>  Just by chance I found out that doing all I/O inc sync-mode does
> prevent the load from going up. Of course, I/O throughput is not
> stellar (but not much worse than the non-O_DIRECT case). But the
> responsiveness seem OK. Maybe a solution, as this can be controlled via
> mount (would be great for O_DIRECT :-).
> 
>  In general 2.6.22 seems to bee better that 2.6.19, but this is highly
> subjective :-( I am using the following setting in /proc. They seem to
> provide the smoothest responsiveness:
> 
> vm.dirty_background_ratio = 1
> vm.dirty_ratio = 1
> vm.swappiness = 1
> vm.vfs_cache_pressure = 1
> 
>  Another thing I saw during my tests is that when writing to NFS, the
> "dirty" or "nr_dirty" numbers are always 0. Is this a conceptual thing,
> or a bug?
> 
>  In any case, view this as a report for one specific loadcase that does
> not behave very well. It seems there are ways to make things better
> (sync, per device throttling, ...), but nothing "perfect yet. Use once
> does seem to be a problem.

Try limiting the queue depth on the cciss device, some of those are
notoriously bad at starving commands. Something like the below hack, see
if it makes a difference (and please verify in dmesg that it prints the
message about limiting depth!):

diff --git a/drivers/block/cciss.c b/drivers/block/cciss.c
index 084358a..257e1c3 100644
--- a/drivers/block/cciss.c
+++ b/drivers/block/cciss.c
@@ -2992,7 +2992,12 @@ static int cciss_pci_init(ctlr_info_t *c, struct pci_dev *pdev)
 		if (board_id == products[i].board_id) {
 			c->product_name = products[i].product_name;
 			c->access = *(products[i].access);
+#if 0
 			c->nr_cmds = products[i].nr_cmds;
+#else
+			c->nr_cmds = 2;
+			printk("cciss: limited max commands to 2\n");
+#endif
 			break;
 		}
 	}

-- 
Jens Axboe

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