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Re: [Xen-devel] a question about popen() performance on domU

See comments below. 

Thanks Mats. I have more questions about your comments below.

Xuehai


-----Original Message-----
From: xuehai zhang [mailto:hai@xxxxxxxxxxxxxxx] Sent: 24 November 2005 14:02 
To: Petersson, Mats
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx; Tim Freeman; Kate Keahey
Subject: Re: [Xen-devel] a question about popen() performance on domU

Mats,

Thanks a lot for the response.
I did have a look at popen, and essentially, it does the
following [
the real code is MUCH more complicated, doing lots of
open/dup/close 
on pipes and stuff]:
if (!fork())
exec("/bin/sh", "sh", "-c", cmd, NULL);
I took a look at the popen source code too yesterday and the above lines are the esstential part. A thread at gnu list (http://lists.gnu.org/archive/html/bug-global/2005-06/msg00001 
.html) suggets
popen() might depend on how fast /bin/sh is executed. On both my VM and the physical machine, the kernel version is 2.6.11, glibc version is 2.3.2.ds1-21, and /bin/sh is linked to /bin/bash. I also tried to see any difference of the shared libraries used by /bin/sh on both machines and found /bin/sh on the physical machine uses libraries from /lib/tls while for the VM this directory is disabled. 

VM$ ldd /bin/sh
        libncurses.so.5 => /lib/libncurses.so.5 (0xb7fa7000)
        libdl.so.2 => /lib/libdl.so.2 (0xb7fa3000)
        libc.so.6 => /lib/libc.so.6 (0xb7e70000)
        /lib/ld-.so.2 => /lib/ld-.so.2 (0xb7fea000)

PHYSICAL$  ldd /bin/sh
        libncurses.so.5 => /lib/libncurses.so.5 (0xb7fa6000)
        libdl.so.2 => /lib/tls/libdl.so.2 (0xb7fa2000)
        libc.so.6 => /lib/tls/libc.so.6 (0xb7e6d000)
        /lib/ld-.so.2 => /lib/ld-.so.2 (0xb7fea000)



In this particular case, I would think that lib/tls is not a factor, but
it may be worth disabling the tls libraries on the pysical machine too,
just to make sure... [just "mv /lib/tls /lib/tls.disabled" should do
it].
I don't think /lib/tls is the factor too. I did rerun the tests with tls disabled on the physical machine and it gave even worse performance for the tests. So, I switched it back.
The fork creates another process, which then executes the /bin/sh, which again causes another fork/exec to take place in the effort of executing the actual command given.
So the major component of popen would be fork() and
execl(), both of
which cause, amongst other things, a lot of page-table work and task-switching.
Note that popen is implemented in glibc [I took the 2.3.6
source code
from www.gnu.org for my look at this], so there's no
difference in the
implementation of popen itself - the difference lies in how
the Linux
kernel handles fork() and exec(), but maybe more importantly, how task-switches and page-tables are handled in Linux native 

and Xen-Linux.
Because Xen keeps track of the page-tables on top of
Linux's handling
of page-tables, you get some extra work here. So, it should
really be 
slower on Xen than on native Linux.
[In fact, the question came up not so long ago, why Xen was SLOWER than native Linux on popen (and some others) in a particular benchmark, and the result of that investigation was that
it's down to, 
mainly, task-switching takes longer in Xen.]
I agree with your explanation about Xen was SLOWER than native Linux on popen because of the longer task-switching in Xen. The problem I met (popen runs faster on Xen VM than the physical machine) looks abnormal. I ran several home-made benchmarking programming and used the "strace" tool to trace the system call performance. The first program is to test the performance of both popen and pclose (a loop of popen call with a followup pclose call) and the source of the program and the strace results are available at http://people.cs.uchicago.edu/~hai/tmp/gt2gram/strace-popen/st race.txt. The results shows the waitpid syscall costs more time on physical machine than on the VM (see the usecs/call valuee in the following table).
% time seconds usecs/call calls errors syscall ------ ----------- ----------- --------- --------- ---------------- VM: 63.43 0.127900 6395 20 waitpid 
PHYSICAL
MACHINE: 93.87 0.532498 26625 20 waitpid
waitpid is called by pclose as shown in the glibc source code. So, my original post questioning the performance of popen should take pclose into consideration too. A more accurate question I should post is, popen+pclose executes faster on my VM than my physical machine. The popen/pclose benchmark I did narrows the problem down to waitpid that waitpid somehow is suffering on the physical machine. So, I did a followup experiment to test the fork and waitpid performance on both machines. The program is a loop of fork call with a followup waitpid call. The source of the program and the strace results are available at http://people.cs.uchicago.edu/~hai/tmp/gt2gram/strace-fork/str ace.txt. The strace results confirm the waitpid costs more time on the physical machine (154 usec/call) than the VM (56 usec/call). However, the program runs faster on the physical machine (not like the popen/pclose program) and the results suggest the fork syscall used on the VM costs more time than the clone syscall on the physical machine. I have a question here, why the physical machine doesn't use fork syscall but the clone syscall for the same program? 


Because it's using the same source for glibc! glibc says to use
_IO_fork(), which is calling the fork syscall. Clone would probably do
the same thing, but for whatever good or bad reason, the author(s) of
thise code chose to use fork. There may be good reasons, or no reason at
all to do it this way. I couldn't say. I don't think it makes a whole
lot of difference if the actual command executed by popen is actually
"doing something", rather than just an empty "return".
Do you have any suggestion why the same code uses different syscalls on two machines which have the same kernel and glibc?
The reason it is not would probably have something to do with the differences in hardware on Linux vs. Xen platforms, perhaps
the fact
that your file-system is a virtual block-device and thus
lives inside
a file that is perhaps better cached or otherwise handled in a different way on the Xen-system.
Let me describe the hardware context of my VM and physical machine. The host of my VM and the physical machine I tested against the VM, are two nodes of a physical cluster with the same hardware configuration (Dual Intel PIII 498.799 MHz CPU, 512MB memory, a 4GB HD with same partitions). The physical machine is rebooted with "nosmp". The VM host is rebooted into Xen with "nosmp" (Xen version information is "Latest ChangeSet: 2005/05/03 17:30:40 1.1846 4277a730mvnFSFXrxJpVRNk8hjD4Vg"). Xen dom0 is assigned 96MB memory and the VM is the only user domain running on the VM host with 395MB memory. Both dom0 and the VM are pinned to CPU 0.
Yes, the backends of the VM's VBDs are loopback files in dom0. Three loopback files are used to map to three partitions inside of the VM. I acutally thought about the possible caching effect of the VM's VBD backends, but not sure how to testify it and compare it with the physical machine. Is it possible the Xen has different assurance of writing back than the physical machine, that is, the data is kept in memory longer before is actually written to disk? 


Xen itself doesn't know ANYTHING about the disk/file where the data for
the Dom0 or DomU comes from, so no, Xen would not do that. However, the
loopback file-system that is involved in VBD's would potentially do
things that are different from the actual hardware.
So, there is possbility that the loopback file-system can do something tricky like caching and results in better performance for applications running inside of the VM? 


I think you should be able to mount the virtual disk as a "device" on
your system. 

What does "your system" here refer to? Does it mean dom0 or inside of domU?


I don't know of the top of my head how to do that, but
essentially something like this:
mount myimage.hdd loop/ -t ext3 [additional parameters may be needed]. 
You could then do "chroot loop/", and perform your tests there. This
should execute the same thing from the same place on the native linux as
you would in DomU. 
Now, this may not run faster on native than your original setup, but I
wouldn't be surprised if it does...
This is interesting. I will try to run the same tests if I canmount the virtual disk as "device" successfully. 

Thanks.

Xuehai
Now, I'm not saying that there isn't a possibility that
something is
managed differently in Xen that makes this run faster - I
just don't
really see how that would be likely, since everything that
happens in
the system is going to be MORE complicated by the extra
layer of Xen 
involved.
If anyone else has some thoughts on this subject, it would be interesting to hear.
I agree. But given the VM having same hardware/software configuration as the physical machine, it runs faster still looks abnormal to me. I wonder if there is any other more efficient debugging strategies I can use to investigate it. I appreciate if any one has any more suggestions. 

Thanks again.

Xuehai



-----Original Message-----
From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
[mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of xuehai zhang 
Sent: 23 November 2005 20:26
To: xen-devel@xxxxxxxxxxxxxxxxxxx
Cc: Tim Freeman; Kate Keahey
Subject: [Xen-devel] a question about popen() performance on domU

Dear all,
When I compared the performance of some application on both
a Xen domU
and a standard linux machine (where domU runs on a similar physical mahine), I notice the application runs faster on the domU
than on the 
physical machine.
Instrumenting the application code shows the application
spends more
time on popen() calls on domU than on the physical machine.
I wonder
if xenlinux does some special modification of the popen code to improve its performance than the original Linux popen code? 
Thanks in advance for your help.
Xuehai

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