-----Original Message-----
From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
[mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Jeremy Fitzhardinge
Sent: Tuesday, December 21, 2010 9:39 AM
To: Dante Cinco
Cc: Xen-devel; Konrad Rzeszutek Wilk
Subject: [Xen-devel] Re: domU using linux-2.6.37-xen-next pvops kernel with
CONFIG_PARAVIRT_SPINLOCKS disabled results in 150% performance improvement
(updated)
On 12/20/2010 05:03 PM, Dante Cinco wrote:
> (Sorry, I accidentally sent the previous post before finishing the
> summary table)
>
> For a couple of months now, we've been trying to track down the slow
> I/O performance in pvops domU. Our system has 16 Fibre Channel
> devices, all PCI-passthrough to domU. We were previously using a
> 2.6.32 (Ubuntu version) HVM kernel and were getting 511k IOPS. We
> switched to pvops with Konrad's xen-pcifront-0.8.2 kernel and were
> disappointed to see the performance degrade to 11k IOPS. After
> disabling some kernel debug options including KMEMLEAK, the
> performance jumped to 186k IOPS but still well below what we were
> getting with the HVM kernel. We tried disabling spinlock debugging in
> the kernel but it actually resulted in a drop in performance to 70k IOPS.
>
> Last week we switched to linux-2.6.37-xen-next and with the same
> kernel debug options disabled, the I/O performance was slightly better
> at 211k IOPS. We tried disabling spinlock debugging again and saw a
> similar drop in performance to 58k IOPS. We searched around for any
> performance-related posts regarding pvops and found two references to
> CONFIG_PARAVIRT_SPINLOCKS (one from Jeremy and one from Konrad):
> http://lists.xensource.com/archives/html/xen-devel/2009-05/msg00660.ht
> ml
> http://lists.xensource.com/archives/html/xen-devel/2010-11/msg01111.ht
> ml
>
> Both posts recommended (Konrad strongly) enabling PARAVIRT_SPINLOCKS
> when running under Xen. Since it's enabled by default, we decided to
> see what would happen if we disabled CONFIG_PARAVIRT_SPINLOCKS. With
> the spinlock debugging enabled, we were getting 205k IOPS but with
> spinlock debugging disabled, the performance leaped to 522k IOPS !!!
>
> I'm assuming that this behavior is unexpected.
Yeah, that would be one way to put it.
>
> Here's a summary of the kernels, config changes and performance (in
> IOPS):
>
> pcifront linux
> 0.8.2 2.6.37-xen-next
> pvops pvops
> Spinlock
> debugging enabled, 186k 205k
> PARAVIRT_SPINLOCKS=y
>
> Spinlock
> debugging disabled, 70k 58k
> PARAVIRT_SPINLOCKS=y
>
> Spinlock
> debugging disabled, 247k 522k
> PARAVIRT_SPINLOCKS=n
Fascinating.
Spinlock debugging ends up bypassing all the paths that PARAVIRT_SPINLOCKS
affects, so that's consistent with the problem being the paravirt locking code.
Basically, there's 3 reasons paravirt spinlocks could slow things down:
1. the overhead of calling into the pv lock code is costing a lot
(very hard to imagine how it would cause this degree of slowdown)
2. you're hitting the spinlock slowpath very often, and end up making
lots of hypercalls
3. your system and/or workload gets a very strong benefit from the
ticket lock's FIFO properties
4. (something else entirely)
When you're running with PARAVIRT_SPINLOCKS=y, are you getting a lot of counts
on the per-cpu spinlock irqs?
What happens if you raise the "timeout" threshold? If you have XEN_DEBUG_FS
enabled, you can do that on the fly by writing it to
/sys/kernel/debug/xen/spinlocks/timeout, or adjust TIMEOUT in
arch/x86/xen/spinlock.c. In theory, if you set it very large it should have
the same effect as just disabling PARAVIRT_SPINLOCKS (except still using byte
locks). That should help isolate which of the possibilities above are coming
into play.
The other data in /sys/kernel/debug/xen/spinlocks could be helpful in working
out what's going on as well.
Do you know if there are specific spinlocks being particularly pounded on by
your workload? I'm guessing some specific to your hardware.
Based on lock statistics we got, the top 5 lock contentions are in our software
stack and the lock contention number are 351375/259924/246715/160125/156188
respectively. (pretty much consistent with top 5 lock contentions when we were
running with hvm kernel) We also note that there is kernel option to
enable/disable Big Kernel Lock in .37 pv-ops. How does this BKL interact with
PARAVIRT_SPINLOCK ? Does disabling BKL affect the performance ?
Thanks,
J
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