Jens Axboe wrote:
On Wed, Aug 19 2009, Jeff Garzik wrote:
On 08/19/2009 07:25 AM, Jens Axboe wrote:
Hi,
On boxes with lots of CPUs, we have so many kernel threads it's not
funny. The basic problem is that create_workqueue() creates a per-cpu
thread, where we could easily get by with a single thread for a lot of
cases.
One such case appears to be ata_wq. You want at most one per pio drive,
not one per CPU. I'd suggest just dropping it to a single threaded wq.
Signed-off-by: Jens Axboe<jens.axboe@xxxxxxxxxx>
diff --git a/drivers/ata/libata-core.c b/drivers/ata/libata-core.c
index 072ba5e..0d78628 100644
--- a/drivers/ata/libata-core.c
+++ b/drivers/ata/libata-core.c
@@ -6580,7 +6580,7 @@ static int __init ata_init(void)
{
ata_parse_force_param();
- ata_wq = create_workqueue("ata");
+ ata_wq = create_singlethread_workqueue("ata");
if (!ata_wq)
goto free_force_tbl;
I agree with one-thread-per-cpu is too much, in these modern multi-core
times, but one thread is too little. You have essentially re-created
simplex DMA -- blocking and waiting such that one drive out of ~4 can be
used at any one time.
ata_pio_task() is in a workqueue so that it can sleep and/or spend a
long time polling ATA registers. That means an active task can
definitely starve all other tasks in the workqueue, if only one thread
is available. If starvation occurs, it will potentially pause the
unrelated task for several seconds.
The proposed patch actually expands an existing problem -- uniprocessor
case, where there is only one workqueue thread. For the reasons
outlined above, we actually want multiple threads even in the UP case.
If you have more than one PIO device, latency is bloody awful, with
occasional multi-second "hiccups" as one PIO devices waits for another.
It's an ugly wart that users DO occasionally complain about; luckily
most users have at most one PIO polled device.
It would be nice if we could replace this workqueue with a thread pool,
where thread count inside the pool ranges from zero to $N depending on
level of thread pool activity. Our common case in libata would be
_zero_ threads, if so...
That would be ideal, N essentially be:
N = min(nr_cpus, nr_drives_that_need_pio);
..
No, that would leave just a single thread again for UP.
It would be nice to just create these threads on-demand,
and destroy them again after periods of dis-use.
Kind of like how Apache does worker threads.
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