RE: [Xen-devel] open/stat64 syscalls run faster on Xen VM than standard Linux

["Petersson, Mats" <mats.petersson@xxxxxxx>]

> -----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. 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. 

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. 

--
Mats
> 
> # 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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