Re: [Xen-devel] Forking time in Xen

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Mon, Jun 18, 2012 at 04:38:39PM +0700, Sergey Zhukov wrote:
> 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?
> 

If you need good fork-performance you should use Xen PVHVM guests, not PV.

Xen PV model needs to validate the new process page tables in the hypervisor 
every time when a fork happens, so that will have some performance hit in 
fork-heavy workloads.

HVM does not need to do that.. so please try switching to Xen PVHVM and 
benchmark again.

-- Pasi


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