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xen-devel
Re: [Xen-devel] open/stat64 syscalls run faster on Xen VM than standard
Petersson, Mats wrote:
-----Original Message-----
From: xuehai zhang [mailto:hai@xxxxxxxxxxxxxxx]
Sent: 28 November 2005 15:51
To: Petersson, Mats
Cc: Anthony Liguori; Xen Mailing List; Kate Keahey; Tim Freeman
Subject: Re: [Xen-devel] open/stat64 syscalls run faster on
Xen VM than standard Linux
Mats,
I mounted the loopback file in dom0, chrooted to the
mountpoint and redid the experiment. The results is attached
below. The time of open and stat64 calls is similar to the
XenLinux case and also much smaller than the standard Linux
case. So, either using loopback file as backend of XenLinux
or directly mounting it in local filesystem will result in
some benefit (maybe just caused by the extra layer of block
caching) for the performance of some system calls.
Yes, I think the caching of the blocks in two layers will be the reason
you get this effect. The loopback file is cached once in the fs handling
the REAL HARD DISK, and then other blocks would be cached in the fs
handling the loopback.
Is "the fs handling the REAL HD" the dom0's filesystem? Is the cache used here is the dom0's disk
buffer cache or something else? What is "the fs handling the loopback"? Is the filesystem seen
inside of the XenLinux or still the filesystem of dom0? What is the cache used in this case?
In this case the directory of the file(s)
involved in your benchmark are probably held entirely in memory, whilst
when you use a real disk to do the same thing, you could end up with
some "real" accesses to the disk device itself.
To confirm our hypothesis that two layer block caching is the real cause, what experiments I can do
to show exactly a block is accessed from a cache instead of hard disk on XenLinux but it has to be
read from hard disk on stand Linux? Maybe I can use "vmstat" in dom0 to track block receive/send
during the execution of the benchmark.
Next question will probably be why write is slower in Xen+Linux than
native Linux - something I can't say for sure, but I would expect it to
be because the write is going through Xen in the Xen+Linux case and
straight through Linux when in the native linux case. But that's just a
guess. [And since it's slower in Xen, I don't expect you to be surprised
by this]. And the write call is almost identical to the Linux native, as
you'd expect.
I also agree the overhead of write system call in VM is caused by Xen. I actually run a "dd"
benchmark to create a disk file from /dev/zero on both machines and the VM is slower than the
physical machine as we expect.
So, the benchmark experiments I've done so far suggests XenLinux using loopback files as VBD
backends shows better performance (faster execution) on part of the system calls like open and
stat64, but it shows worse performance (slower execution) on other system calls like write than the
standard Linux. Does this mean different applications may have different execution behaviors on VM
than on the standard Linux? In other words, some applications run faster on VM and some slower,
comparing with the physical machine?
Thanks.
Xuehai
# strace -c /bin/sh -c /bin/echo foo
% time seconds usecs/call calls errors syscall
------ ----------- ----------- --------- --------- ----------------
21.93 0.000490 490 1 write
16.34 0.000365 24 15 old_mmap
15.26 0.000341 38 9 3 open
9.62 0.000215 43 5 read
7.97 0.000178 10 18 brk
7.79 0.000174 87 2 munmap
4.07 0.000091 8 11 rt_sigaction
3.27 0.000073 12 6 close
2.91 0.000065 11 6 fstat64
2.28 0.000051 9 6 rt_sigprocmask
2.15 0.000048 24 2 access
1.75 0.000039 13 3 uname
1.66 0.000037 19 2 stat64
0.40 0.000009 9 1 getpgrp
0.40 0.000009 9 1 getuid32
0.36 0.000008 8 1 time
0.36 0.000008 8 1 getppid
0.36 0.000008 8 1 getgid32
0.31 0.000007 7 1 getpid
0.27 0.000006 6 1 execve
0.27 0.000006 6 1 geteuid32
0.27 0.000006 6 1 getegid32
------ ----------- ----------- --------- --------- ----------------
100.00 0.002234 95 3 total
Thanks.
Xuehai
Petersson, Mats wrote:
-----Original Message-----
From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
[mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Anthony
Liguori
Sent: 28 November 2005 14:39
To: xuehai zhang
Cc: Xen Mailing List
Subject: Re: [Xen-devel] open/stat64 syscalls run faster on Xen VM
than standard Linux
This may just be the difference between having the extra level of
block caching from using a loop back device.
Try running the same benchmark on a domain that uses an actual
partition. While the syscalls may appear to be faster, I
imagine it's
because the cost of pulling in a block has already been
payed so the
overall workload is unaffected.
And this would be the same as running standard linux with
the loopback
file-system mounted and chroot to the local file-system, or
would that
be different? [I'm asking because I don't actually
understand enough
about how it works to know what difference it makes, and I
would like
to know, because at some point I'll probably need to know this.]
--
Mats
Regards,
Anthony Liguori
xuehai zhang wrote:
[snip]
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