I've analysed the problem with migration using perf-events, and it is confirmed
cost of glibc is significantly increased.
I made a simple test code to execute read system call on the guest as follows,
and examined the performance from host and guest.
fd = open("/dev/zero", O_RDONLY);
while (1) { read(fd &ch, 1); }
---
[Source host]
$ sudo perf kvm --host --guest record -a -o src_host.perf sleep 10
Events: 10K cycles
89.36% qemu-system-x86 [unknown] [g] 0xffffffff810a36ef
9.04% qemu-system-x86 [unknown] [u] 0x3fd20e2020
0.16% swapper [kernel.kallsyms] [k] intel_idle
0.11% sleep [kernel.kallsyms] [k] page_fault
[Guest on source host]
$ perf record -o src_guest.perf ./loop_read
Events: 29K cpu-clock
11.71% loop_read [kernel.kallsyms] [k] system_call_after_swapgs
9.58% loop_read libc-2.14.90.so [.] __GI___libc_read
6.92% loop_read [kernel.kallsyms] [k] vfs_read
5.53% loop_read [kernel.kallsyms] [k] fsnotify
__GI___libc_read
: 0000003fd20e2010 <__read>:
2.31 : 3fd20e2010: 83 3d 1d 22 2d 00 00 cmpl $0x0,0x2d221d(
5.67 : 3fd20e2017: 75 10 jne 3fd20e2029 <__
:
: 0000003fd20e2019 <__read_nocancel>:
1.82 : 3fd20e2019: b8 00 00 00 00 mov $0x0,%eax
0.00 : 3fd20e201e: 0f 05 syscall
87.78 : 3fd20e2020: 48 3d 01 f0 ff ff cmp $0xfffffffffff
0.00 : 3fd20e2026: 73 31 jae 3fd20e2059 <__
2.42 : 3fd20e2028: c3 retq
[Destination host]
$ sudo perf kvm --host --guest record -a -o dst_host.perf sleep 10
Events: 10K cycles
58.39% qemu-system-x86 [unknown] [g] 0xffffffff810a3a6e
40.14% qemu-system-x86 [unknown] [u] 0x3fd20e2017
0.13% gnome-shell nouveau_dri.so [.] 0xbd7c9
0.11% swapper [kernel.kallsyms] [k] intel_idle
[Guest on destination host]
$ perf record -o dst_guest.perf ./loop_read
Events: 29K cpu-clock
41.95% loop_read libc-2.14.90.so [.] __GI___libc_read
7.90% loop_read [kernel.kallsyms] [k] system_call_after_swapgs
4.61% loop_read [kernel.kallsyms] [k] vfs_read
3.72% loop_read [kernel.kallsyms] [k] fsnotify
__GI___libc_read
: 0000003fd20e2010 <__read>:
0.41 : 3fd20e2010: 83 3d 1d 22 2d 00 00 cmpl $0x0,0x2d221d(
86.10 : 3fd20e2017: 75 10 jne 3fd20e2029 <__
:
: 0000003fd20e2019 <__read_nocancel>:
0.33 : 3fd20e2019: b8 00 00 00 00 mov $0x0,%eax
0.00 : 3fd20e201e: 0f 05 syscall
12.84 : 3fd20e2020: 48 3d 01 f0 ff ff cmp $0xfffffffffff
0.00 : 3fd20e2026: 73 31 jae 3fd20e2059 <__
0.33 : 3fd20e2028: c3 retq
---
After the migration, "jne" instruction in __read became to take a long time.
This is due to increase the overhead of read system call. The performance was
not improved even if thp/hugetlb was disabled.
Does this problem happen only to me? Should I also ask other community about
this problem?
Shota
> -----Original Message-----
> From: Xiao Guangrong [mailto:xiaoguangrong@xxxxxxxxxxxxxxxxxx]
> Sent: Thursday, November 01, 2012 1:45 PM
> To: Uehara, Shouta (Shouta.Uehara@xxxxxxxxxxxxxxx)
> Cc: kvm@xxxxxxxxxxxxxxx
> Subject: Re: Guest performance is reduced after live migration
>
> Shouta,
>
> Can it be reproduced if thp/hugetlbfs is disabled on both source and
> destination?
>
> On 11/01/2012 08:12 AM, Shouta.Uehara@xxxxxxxxxxxxxxx wrote:
> > Hello.
> >
> > I have a problem with the performance of the guest Linux after live
> migration.
> > When I analyze the file I/O latency of the guest using LMbench3, the
> > latency of the guest on the destination host is about 2 times bigger
> > than the guest on the source host. As a result that I investigated it,
> > this problem occurs if three following conditions are right.
> >
> > 1. Use the kernel version 2.6.38.6-26.fc15.x86_64 or later.
> > 2. Execute system calls for low level file I/O (read, write, open etc.)
> on the
> > guest.
> > 3. Enable EPT.
> >
> > Performance cannot decrease on other tests of LMbench3; numerical
> > operation, context switching, memory latencies etc.
> >
> > Is this an existing problem ? What should I do to resolve it ?
> >
> > ---
> > The situation the problem occurs
> >
> > CPU models : Intel Xeon E3-1245 (Both hosts have same cpu.)
> > Host kernel : 3.6.4
> > Host OS : Fedora 16 64bit ver. (Both hosts have same OS.)
> > Guest OS : Fedora 16 64bit ver., kernel 3.1.0-7.fc16.x86_64
> > QEMU version : qemu-kvm 1.2.0
> > QEMU parameter : qemu-kvm -m 1024 -drive file=guest.qcow2,if=virtio \
> > -boot order=c,menu=on -k ja
> > (-incoming tcp:192.168.0.51:4444 set only
> destination)
> > QEMU command : migrate -d tcp:192.168.0.51:4444
> > Analyses : LMbench version 3
> >
> > [Guest on the source host]
> > $ ./lat_syscall open
> > Simple open/close: 1.4281 microseconds
> > $ ./lat_syscall read
> > Simple read: 0.1416 microseconds
> > $ ./lat_syscall write
> > Simple write: 0.1194 microseconds
> > $ ./lat_fs -s 0
> > 0k 550 98726 247712
> > 1k 383 69751 193985
> > 4k 381 69348 193383
> > 10k 313 54369 155697
> >
> > [Guest on the source host]
> > $ ./lat_syscall open
> > Simple open/close: 1.6126 microseconds
> > $ ./lat_syscall read
> > Simple read: 0.2206 microseconds
> > $ ./lat_syscall write
> > Simple write: 0.1980 microseconds
> > $ ./lat_fs -s 0
> > 0k 427 83252 243222
> > 1k 315 60530 191650
> > 4k 184 34535 190947
> > 10k 65 11922 152608
> > ---
> >
> > Thanks.
> >
> > Shota Uehara <Shouta.Uehara()jp.yokogawa.com>
> >
> >
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