[Xen-devel] Forking time in Xen

[Sergey Zhukov <svg@xxxxxx>]

Hi,

I repost this message from xen-users list following by others
subscribers suggestions:


I found an article about forking time for redis NoSQL database in
different systems:

http://redis.io/topics/latency
---------Quote------------------
Fork time in different systems
Modern hardware is pretty fast to copy the page table, but Xen is not.
The problem with Xen is not virtualization-specific, but Xen-specific.
For instance using VMware or Virutal Box does not result into slow fork
time. The following is a table that compares fork time for different
Redis instance size. Data is obtained performing a BGSAVE and looking at
the latest_fork_usec filed in the INFO command output.

      * Linux beefy VM on VMware 6.0GB RSS forked in 77 milliseconds
        (12.8 milliseconds per GB).
      * Linux running on physical machine (Unknown HW) 6.1GB RSS forked
        in 80 milliseconds (13.1 milliseconds per GB)
      * Linux running on physical machine (Xeon @ 2.27Ghz) 6.9GB RSS
        forked into 62 millisecodns (9 milliseconds per GB).
      * Linux VM on 6sync (KVM) 360 MB RSS forked in 8.2 milliseconds
        (23.3 millisecond per GB).
      * Linux VM on EC2 (Xen) 6.1GB RSS forked in 1460 milliseconds
        (239.3 milliseconds per GB).
      * Linux VM on Linode (Xen) 0.9GBRSS forked into 382 millisecodns
        (424 milliseconds per GB).

As you can see a VM running on Xen has a performance hit that is between
one order to two orders of magnitude. We believe this is a severe
problem with Xen and we hope it will be addressed ASAP.
----------End of quote-----------------

I made my own test with Xen 4.1 and Redis 2.4 with 7.04GB dataset. The
test was performed on Intel Core I5 2500 processor unit. Forking time
was about 1 sec or 151 ms/GB - it's faster then tests over Amazon
EC2/Linode were mentioned in the article, but still much slower then
VMWare or physical machines. Has anyone running with this issue? Or may
be there is a way to tune Xen for less forking times?

Sergey Zhukov


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