CentOS virt - Feb 2008 - Xen performance (very slow) problem with 4GB of RAM.

I just took the x64 CentOS 5.1/Xen system I've been working on and 
upgraded from 2GB to 4GB of memory. Fresh minimal install of the OS (to 
remove some 3.2 stuff I had done trying to make PCI passthrough work). 
No special options, just a flat out default install.

Dom0 is incredibly slow, I haven't tried DomU's yet to see if they are 
affected. X in particular is impressively slow (you can watch the screen 
be filled in one little block at a time, with several seconds between 
blocks) but it's not just video apps, it takes a considerable amount of 
time for the Xend daemon to start during boot. Before worrying about it 
I updated everything, same problem. Nothing interesting looking in dmesg 
or /var/log/messages.

I tried out the vanilla kernel and it booted/ran well, no sign of the 
problem.

Next, I tried Dom0 limited to 1.5GB (kernel /xen.gz-2.6.18-53.1.6.el5 
dom0_mem=1536M), and it performed well.

This workaround is probably adequate for me, I haven't found what the 
boundary conditions are on the problem and I haven't tested DomU 
performance yet. A Google and BugTracker search didn't turn this problem 
described elsewhere, is this known or is this something unique to me?

Thanks
-- Michael

On Feb 3, 2008 10:23 AM, Michael Olson <olson at irinim.net>
wrote:
>
> I just took the x64 CentOS 5.1/Xen system I've been working on and
> upgraded from 2GB to 4GB of memory. Fresh minimal install of the OS (to
> remove some 3.2 stuff I had done trying to make PCI passthrough work).
> No special options, just a flat out default install.
>
> Dom0 is incredibly slow, I haven't tried DomU's yet to see if they
are
> affected. X in particular is impressively slow (you can watch the screen
> be filled in one little block at a time, with several seconds between
> blocks) but it's not just video apps, it takes a considerable amount of
> time for the Xend daemon to start during boot. Before worrying about it
> I updated everything, same problem. Nothing interesting looking in dmesg
> or /var/log/messages.
>
> I tried out the vanilla kernel and it booted/ran well, no sign of the
> problem.
>
> Next, I tried Dom0 limited to 1.5GB (kernel /xen.gz-2.6.18-53.1.6.el5
> dom0_mem=1536M), and it performed well.
>
> This workaround is probably adequate for me, I haven't found what the
> boundary conditions are on the problem and I haven't tested DomU
> performance yet. A Google and BugTracker search didn't turn this
problem
> described elsewhere, is this known or is this something unique to me?
Hmm the only time I have seen this with regular systems on an X86_64
box was where the system was having a hard time mapping to the higher
memory (especially on AMD boxes where they route to each 'bank'
through a method my brain has conveniently forgot as a started posting
:(.)

1) Make sure that you have the latest system bios for your motherboard.
2) Check your BIOS settings on memory. There were a couple on an HP I
played with a while back that really affected memory performance in 64
bit mode).



-- 
Stephen J Smoogen. -- CSIRT/Linux System Administrator
How far that little candle throws his beams! So shines a good deed
in a naughty world. = Shakespeare. "The Merchant of Venice"