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

[xuehai zhang <hai@cs.uchicago.edu>]

Anthony,

Thanks a lot for the reply.

> This may just be the difference between having the extra level of block 
> caching from using a loop back device.

How can I know exactly how the extra level of block caching is handled in the loop back device? Is 
there anyway to disable the extra block caching when 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.

I will try to test with a domain using actual partitions and report the results.

Thanks again.

Xuehai

> 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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> 
>