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xen-devel
RE: [Xen-devel] open/stat64 syscalls run faster on Xen VM than standard
> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
> [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of
> xuehai zhang
> Sent: 29 November 2005 12:40
> To: Anthony Liguori
> Cc: Kate Keahey; Tim Freeman; Xen Mailing List
> Subject: Re: [Xen-devel] open/stat64 syscalls run faster on
> Xen VM than standard Linux
>
> Anthony Liguori wrote:
> > 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.
>
> I created a new domainU by using the physical partition
> instead of the loopback file as the backends of the VBDs and
> I reran the "strace -c /bin/sh -c /bin/echo foo" benchmark
> inside of the domU. The following are the results. Comparing
> with the results for domU with loopback files as VBDs I
> reported in the previous email (quoted below), the average
> time of open/stat64 syscalls are very similar, but still much
> smaller than the values for standard Linux. If the reason
> that open/stat64 run faster on domU with loopback files as
> VBDs is because of the extra level of block caching from
> using a loop back device, why open/stat64 still run
> similarily faster on domU with physical partition as VBDs
> when there is no extra level of block caching from using a
> loop back device?
Again, have you run the same test-case on the same physical partition?
You should be able to do the same trick with mount and chroot for this
one too.
This is just to rule out other variations (where in the file-system
different files live, just as an example).
I'm also not entirely convinced that you don't get some double-caching
with this method too - there is the real hard-disk driver in Dom0, then
the virtual disk driver in DomU, and the file-system driver in DomU. I
don't know if the real or virtual block-device driver itself will cache
things or not. Someone who knows may want to comment on this...
Still, I agree with Anthony - run some REAL loads on the system, rather
than microbenchmark a particular, very small function of the whole
system. Microbenchmarks are very good at showing how long some
particular thing takes, but they are also very sensitive to extremely
small changes in the system architecture, which for a bigger load would
not be noticable.
I'm doing my best to explain things here, but I'm not sure if I'm
getting the point across, as it seems like you're trying very hard to
get back to the same original question. We do know that Xen behaves
different than Linux. One reason for this is that it's dealing with
hardware devices through a virtual device. This will most of the time
slow things down, but as you've seen, for example due to the
double-caching effect, you may sometimes actually gain from it,
particularly with very small workloads.
--
Mats
>
> XenLinux (physical partition as VBDs)
>
> root@cctest1:~/c# strace -c /bin/sh -c /bin/echo foo
> % time seconds usecs/call calls errors syscall
> ------ ----------- ----------- --------- --------- ----------------
> 39.56 0.001955 1955 1 write
> 18.94 0.000936 936 1 execve
> 7.65 0.000378 24 16 old_mmap
> 7.57 0.000374 42 9 2 open
> 6.27 0.000310 52 6 read
> 5.10 0.000252 84 3 munmap
> 4.92 0.000243 9 26 brk
> 1.92 0.000095 14 7 close
> 1.78 0.000088 8 11 rt_sigaction
> 1.40 0.000069 10 7 fstat64
> 1.01 0.000050 8 6 rt_sigprocmask
> 0.93 0.000046 23 2 access
> 0.79 0.000039 13 3 uname
> 0.69 0.000034 17 2 stat64
> 0.38 0.000019 19 1 ioctl
> 0.16 0.000008 8 1 getppid
> 0.16 0.000008 8 1 getpgrp
> 0.14 0.000007 7 1 time
> 0.14 0.000007 7 1 getuid32
> 0.14 0.000007 7 1 getgid32
> 0.12 0.000006 6 1 getpid
> 0.12 0.000006 6 1 getegid32
> 0.10 0.000005 5 1 geteuid32
> ------ ----------- ----------- --------- --------- ----------------
> 100.00 0.004942 109 2 total
>
> Thanks.
>
> 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
> >>
> >> _______________________________________________
> >> Xen-devel mailing list
> >> Xen-devel@xxxxxxxxxxxxxxxxxxx
> >> http://lists.xensource.com/xen-devel
> >>
> >
> >
>
>
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