Attachment 314574 Details for Bug 459527 – a patch for the problem

[patch] a patch for the problem

linux-2.6.18-optimize-generic_unplug_device.patch (text/plain), 1.61 KB, created by Mikuláš Patočka on 2008-08-19 21:07:48 UTC

(hide)

Description:

Filename:

MIME Type:

Creator: Mikuláš Patočka

Created: 2008-08-19 21:07:48 UTC

Size: 1.61 KB

patch

obsolete

>Upstream commit: dbaf2c003e151ad9231778819b0977f95e20e06f (2.6.26-rc2)
>
>Mike Anderson was doing an OLTP benchmark on a computer with 48 physical
>disks mapped to one logical device via device mapper.
>
>He found that there was a slowdown on request_queue->lock in function
>generic_unplug_device. The slowdown is caused by the fact that when some
>code calls unplug on the device mapper, device mapper calls unplug on all
>physical disks. These unplug calls take the lock, find that the queue is
>already unplugged, release the lock and exit.
>
>With the below patch, performance of the benchmark was increased by 18%
>(the whole OLTP application, not just block layer microbenchmarks).
>
>So I'm submitting this patch for upstream. I think the patch is correct,
>because when more threads call simultaneously plug and unplug, it is
>unspecified, if the queue is or isn't plugged (so the patch can't make
>this worse). And the caller that plugged the queue should unplug it
>anyway. (if it doesn't, there's 3ms timeout).
>
>
>diff -p -u -r linux-2.6.18.x86_64.p3/block/ll_rw_blk.c linux-2.6.18.x86_64/block/ll_rw_blk.c
>--- linux-2.6.18.x86_64.p3/block/ll_rw_blk.c	2008-08-18 14:06:39.000000000 +0200
>+++ linux-2.6.18.x86_64/block/ll_rw_blk.c	2008-08-19 02:31:39.000000000 +0200
>@@ -1681,9 +1681,11 @@ EXPORT_SYMBOL(__generic_unplug_device);
>  **/
> void generic_unplug_device(request_queue_t *q)
> {
>-	spin_lock_irq(q->queue_lock);
>-	__generic_unplug_device(q);
>-	spin_unlock_irq(q->queue_lock);
>+	if (blk_queue_plugged(q)) {
>+		spin_lock_irq(q->queue_lock);
>+		__generic_unplug_device(q);
>+		spin_unlock_irq(q->queue_lock);
>+	}
> }
> EXPORT_SYMBOL(generic_unplug_device);
>

Actions: View | Diff

Attachments on bug 459527: 314574