RE: [Xen-devel] Benchmarking Xen (results and questions)

["Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>]

> I had initially tried BVT; however, at that time it would 
> crash the system if I tried to change any of the settings, I 
> do not know if this is currently the issue.

Any scheduling period more than 50ms really shouldn't result in any
significant context switch overhead.

I suspect what's going on in your tests is that dom0 (that is doing all
the IO) is being being CPU starved. Putting it on a separate hyperthread
will certainly help confirm this diagnosis. [hyperthreading is very
helpful to Xen, and by default it dedicates the first hyperthread of the
first CPU to dom0]
 
> Also, if you have time, could you elaborate on my WebBench results?

It would be useful if you could explain a bit more about your webbench
setup, e.g. are you testing the clietns rather than the web server?

Best,
Ian

> Thanks,
> David
> 
> -----Original Message-----
> From: Ian Pratt [mailto:m+Ian.Pratt@xxxxxxxxxxxx]
> Sent: Wednesday, August 03, 2005 6:48 PM
> To: Wolinsky, David; xen-devel@xxxxxxxxxxxxxxxxxxx
> Cc: ian.pratt@xxxxxxxxxxxx
> Subject: RE: [Xen-devel] Benchmarking Xen (results and questions)
> 
> 
> David,
> 
> Which xen version is this? I'm guessing unstable.
> Is this with sedf or bvt? I'm guessing sedf since you're 
> playing around
> with periods.
> 
> It would be interesting to retry a couple of datapoints with sched=bvt
> on the xen command line.
> 
> Also, I'd definitely recommend enabling HyperThreading and dedicating
> one of the logical CPUs to dom0.
> 
> Also, are you sure the drop-off in performance isn't just 
> caused because
> of the reduced memory size when you have more VMs? It's 
> probably better
> to do such experiments with the same memory size throughout.
> 
> Best,
> Ian
>  
> 
> > -----Original Message-----
> > From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
> > [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> > David_Wolinsky@xxxxxxxx
> > Sent: 04 August 2005 00:21
> > To: xen-devel@xxxxxxxxxxxxxxxxxxx
> > Subject: [Xen-devel] Benchmarking Xen (results and questions)
> > 
> > Hi all,
> > 
> > Here are some benchmarks that I've done using Xen. 
> > 
> > However, before I get started, let me explain some of configuration 
> > details...
> > 
> > Xen Version     SPECjbb
> >         WebBench       
> > Linux Distribution      Debian 3.1     
> > HT      disabled       
> > Linux Kernel    2.6.12.2       
> > Host Patch      CK3s   
> > 
> > 
> > Here are the initial benchmarks
> > 
> >         SPECJBB WebBench                               
> >         1 Thread        1 Client        2 Clients       4 
> > Clients       8 Clients      
> >         BOPS    TPS     TPS     TPS     TPS    
> > Host    32403.5 213.45  416.86  814.62  1523.78
> > 1 VM    32057   205.4   380.91  569.24  733.8  
> > 2 VM    24909.25        NA      399.29  695.1   896.04 
> > 4 VM    17815.75        NA      NA      742.78  950.63 
> > 8 VM    10216.25        NA      NA      NA      1002.81
> > 
> > 
> > (and some more notes.... BOPS - business operations per 
> second, TPS - 
> > transactions per second...
> > SPECjbb tests CPU and Memory
> > WebBench (the way we configured it) tests Network I/O and Disk I/O
> > 
> > Values = AVG * VM count        
> > Domain configurations          
> >         1 VM - 1660 MB - SPECJBB 1500MB
> >         2 VM - 1280 MB - SPECJBB - 1024MB      
> >         4 VM - 640 MB - SPECJBB - 512 MB       
> >         8 VM - 320 MB - SPECJBB  - 256 MB      
> > 
> > Seeing how the SPECjbb numbers declined so bizarrely, I did some 
> > scheduling tests and found this out...
> > 
> > Test1:  Examine Xen's scheduling to determine if context 
> > switching is causing the overhead                           
>           
> >                 Period  Slice   BOPs   
> > Modified        8 VM    1 ms    125 us  6858   
> >         8 VM    10 ms   1.25 ms 14287  
> >         8 VM    100 ms  12.5 ms 18912  
> >         8 VM    1 Sec   .125 Sec        20695  
> >         8 VM    2 Sec   .25 Sec 21072  
> >         8 VM    10 Sec  1.25 Sec        21797  
> >         8 VM    100 Sec 12.5 Sec        11402  
> > 
> > I later learned that there was a period limit of 4 seconds, thus 
> > invalidating 10 and 100 seconds.  However, this graph suggests that 
> > Xen needs some load and scheduling balancing done.
> > 
> > I also did a memory test to determine if that could be the 
> issue... I 
> > made a custom stream to run for a 2 minute period...
> > and got these numbers
> > 
> >                 Copy    Scale   Add     Triad  
> > Host            3266.4  3215.47 3012.28 3021.79
> > Modified        1 VM    3262.34 3220.34 3016.13 3025.28
> > 
> > 
> > So we can see memory is not the issue... 
> > 
> > Now onto WebBench - After comparing the WebBench to the SPECjbb 
> > results, we get something interesting... NUMBERS increase as we 
> > increase the virtual machien count... So I would really 
> like some idea
> 
> > on why this is.  My understanding is this...  When using the shared 
> > memory network drivers, there must be a local buffer, and when the 
> > buffer fills up, it puts the remaining into a global 
> buffer, and when 
> > that fills up it puts it into a disk buffer?  (These are all 
> > assumptions please correct me...)  If that is the case is there an 
> > easy way to increase the local buffer to attempt to get better 
> > numbers?  I also am looking into doing some tests that deal with 
> > multiple small transactions and 1 large transactions...  I 
> ran these 
> > all against a physical and image backed disk.
> > Please any suggestions.
> > 
> > (Note... I was running this on a 1 gigabit switch with only 
> webbench 
> > running)...
> > 
> > If there are any questions, I would be glad to respond. 
> > 
> > Thanks,
> > David
> > 
> > 
> 

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