[Xen-devel] domU using linux-2.6.37-xen-next pvops kernel with CONFIG_PARAVIRT_SPINLOCKS disabled results in 150% performance improvement

[Dante Cinco <dantecinco@xxxxxxxxx>]

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.html
http://lists.xensource.com/archives/html/xen-devel/2010-11/msg01111.html

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 !!!

So here's a summary of the kernels, config changes and performance (in IOPS

                      2.6.32   pcifront   linux

                      Ubuntu   0.8.2      2.6.37-xen-next

                      HVM      pvops      pvops

Spinlock

debugging enabled,     511k     186k       205k

PARAVIRT_SPINLOCKS=n

Spinlock

debugging enabled,

PARAVIRT_SPINLOCKS=y

Spinlock

debugging disabled,

PARAVIRT_SPINLOCKS=n

                        Performance (in thousands IOPS)

                        kernel debug    spinlock disabled       spinlock disabled

                        disabled        PARAVIRT_SPINLOCKS=y    PARAVIRT_SPINLOCKS not set
2.6.32 Ubuntu HVM       511             

pcifront-0.8.2          186             70          

linux-2.6.37-xen-next   205             58                      522

system: HP Proliant DL380G7

Xen: 4.1-unstable (Thu Oct 14 12:46:29 2010 +0100 22241:3a5755249361)

dom0: 2.6.32.24-pvops-stable (xen/stable)

domU: linux-2.6.37-xen-next (2.6.37-rc5-pvops)

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