Re: open/stat64 syscalls run faster on Xen VM than standard Linux

[Anthony Liguori <aliguori@us.ibm.com>]

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.

Regards,

Anthony Liguori

xuehai zhang wrote:

> Dear all,
> When I debugged the execution performance of an application using 
> strace, I found there are some system calls like open and stat64 which 
> run faster on XenLinux than the standard Linux. The following is the 
> output of running "strace -c /bin/sh -c /bin/echo foo" on both 
> systems. An open call runs averagely 109 usec on standard Linux but 
> only 41 usecs on XenLinux. An stat64 call runs
> 75 usecs on standard Linux but only 19 usecs on XenLinux.
> The Xen VM runs on the same physical machine as the standard Linux. It 
> uses loopback files in dom0 as the backends of VBDs.
> Any insight is highly appreciated.
> Thanks.
> Xuehai
>
> XenLinux:
> # strace -c /bin/sh -c /bin/echo foo
>
> % time     seconds  usecs/call     calls    errors syscall
> ------ ----------- ----------- --------- --------- ----------------
>  39.05    0.001972        1972         1           write
>  19.35    0.000977         977         1           execve
>   7.74    0.000391          24        16           old_mmap
>   7.23    0.000365          41         9         2 open
>   6.06    0.000306          51         6           read
>   5.17    0.000261          10        26           brk
>   4.93    0.000249          83         3           munmap
>   1.98    0.000100          14         7           close
>   1.90    0.000096           9        11           rt_sigaction
>   1.52    0.000077          11         7           fstat64
>   1.01    0.000051           9         6           rt_sigprocmask
>   0.95    0.000048          24         2           access
>   0.81    0.000041          14         3           uname
>   0.75    0.000038          19         2           stat64
>   0.38    0.000019          19         1           ioctl
>   0.18    0.000009           9         1           time
>   0.18    0.000009           9         1           getppid
>   0.16    0.000008           8         1           getpgrp
>   0.16    0.000008           8         1           getuid32
>   0.14    0.000007           7         1           getgid32
>   0.12    0.000006           6         1           getpid
>   0.12    0.000006           6         1           geteuid32
>   0.12    0.000006           6         1           getegid32
> ------ ----------- ----------- --------- --------- ----------------
> 100.00    0.005050                   109         2 total
>
>
> Standard Linux:
> # strace -c /bin/sh -c /bin/echo foo
>
> % time     seconds  usecs/call     calls    errors syscall
> ------ ----------- ----------- --------- --------- ----------------
>  22.90    0.000982         109         9         2 open
>  22.85    0.000980         980         1           execve
>  10.87    0.000466          29        16           old_mmap
>  10.45    0.000448         448         1           write
>   7.06    0.000303          51         6           read
>   6.67    0.000286          10        30           brk
>   3.61    0.000155          78         2           access
>   3.50    0.000150          75         2           stat64
>   2.91    0.000125          42         3           munmap
>   2.24    0.000096          14         7           close
>   2.12    0.000091          13         7           fstat64
>   1.84    0.000079           7        11           rt_sigaction
>   1.03    0.000044           7         6           rt_sigprocmask
>   0.72    0.000031          10         3           uname
>   0.19    0.000008           8         1           geteuid32
>   0.16    0.000007           7         1           time
>   0.16    0.000007           7         1           getppid
>   0.16    0.000007           7         1           getpgrp
>   0.16    0.000007           7         1           getuid32
>   0.14    0.000006           6         1           getpid
>   0.14    0.000006           6         1           getgid32
>   0.12    0.000005           5         1           getegid32
> ------ ----------- ----------- --------- --------- ----------------
> 100.00    0.004289                   112         2 total
>
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