Re: [Xen-devel] PV performance degraded after live migration

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 15/03/17 11:20, Olaf Hering wrote:
> After reports about degraded performance after a PV domU was migrated
> from one dom0 to another it turned out that this issue happens with
> every version of Xen and every version of domU kernel.
>
> The used benchmark is 'sysbench memory'. I hacked it up to show how long
> the actual work takes, and that loop takes longer to execute after the
> domU is migrated. In my testing the loop (memory_execute_event) takes
> about 1200ns, after migration it takes about 1500ns. It just writes 0 to
> an array of memory. In total sysbench reports 6500 MiB/sec, after
> migration its just 3350 MiB/sec.
> The source of the modified test can be found here:
> https://github.com/olafhering/sysbench/compare/master...pv
>
> This happens on several hosts. NUMA or not makes no difference. CPU
> pinning or not makes no difference. Pinning of the test pthreads makes
> no difference. It was initially reported with xen-4.4, I see it with
> staging too. The guest kernel makes no difference, several variants of
> xenlinux or pvops based show the slowdown. Also live migration to
> localhost is affected.
>
> The domU.cfg looks like that:
> name='pv'
> memory=1024
> vcpus=4
> cpus=[ "4", "5", "6", "7" ]
> disk=[ 'file:/disk0.raw,xvda,w', ]
> vif=[ 'bridge=br0,type=netfront', ]
> vfb=[ 'type=vnc,vncunused=1,keymap=de', ]
> serial="pty"
> kernel="/usr/lib/grub2/x86_64-xen/grub.xen"
>
> Xen is booted with "console=com1 com1=115200 loglvl=all guest_loglvl=all
> dom0_max_vcpus=2 dom0_vcpus_pin=on".
>
> I wonder what the cause might be, and how to check where the time is
> spent.

As a crazy idea, doest this help?

diff --git a/tools/libxc/xc_sr_common_x86.c b/tools/libxc/xc_sr_common_x86.c
index 98f1cef..ee8089d 100644
--- a/tools/libxc/xc_sr_common_x86.c
+++ b/tools/libxc/xc_sr_common_x86.c
@@ -34,7 +34,7 @@ int handle_tsc_info(struct xc_sr_context *ctx, struct
xc_sr_record *rec)
     }
 
     if ( xc_domain_set_tsc_info(xch, ctx->domid, tsc->mode,
-                                tsc->nsec, tsc->khz, tsc->incarnation) )
+                                tsc->nsec, tsc->khz, 0 /*
tsc->incarnation */) )
     {
         PERROR("Unable to set TSC information");
         return -1;


~Andrew

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