[OpenAFS] Performance

[emoy@apple.com]

As I understand it, the cache helps reads primarily, though if you are  
doing random updates of a file, those also get cached, until you do the  
finally close, which then writes all to the server.  That assumes the  
file fits in the cache.  If not, then a write can force the cache to  
partially empty, so the latest write can be cached.

But there seems to be some pathological behavior, because doing a  
sequential write should ideally not be affected that much by size of  
the cache.  But in a test I did with a 1 GB file, with regular network  
traffic, it took almost 5 minute to write with with a 2 GB cache (about  
10% slower than NFS in this case), but took almost 11 minutes with a  
0.5 GB cache.  The system CPU time rose from 20 seconds for the 2 GB  
cache to 517 seconds with the 0.5 GB cache.

As for being slower than NFS in this test case, with such a large  
cache, tuning the cache parameters does help.  Setting the chucksize to  
256KB dropped the 2 GB cache speed to about 3.5 minutes, beating NFS.
------------------------------------------------------------------------ 
--
Edward Moy
Apple Computer, Inc.
emoy@apple.com

(This message is from me as a reader of this list, and not a statement
from Apple.)

On Thursday, February 27, 2003, at 11:51  AM, Nathan Ward wrote:

> Well I'd expect that it goes slower as your cache size is exceeded as  
> it then needs to start getting that data to the server. Or is the  
> cache for read operations only?
>
> I notice that there are around about the same number of packets/sec as  
> context switches/sec on my client machines. I wonder if switches  
> between userland and kernel could be to blame... ? Who sends packets  
> in OpenAFS, the userspace daemon or the kernel?
>
> Nathan
>
> emoy@apple.com wrote:
>
>> Could the slowness you see with your dd write test be related to the   
>> cache exhaustion issue that I raised recently, when writing a file   
>> larger than your cache size.  Your test writes a 1 GB file, so if  
>> your  cache is smaller than this, you will see poor performance once  
>> your  cache size is exceeded.
>>
>> On Thursday, February 27, 2003, at 11:15  AM, Nathan Ward wrote:
>>
>>> I see pretty bad performance to tell you the truth.
>>> I can read and write ~60mb/s directly to my raid array, but when  
>>> using  OpenAFS (locally or remotely) to the same array, I get around  
>>>  6-10MB/s, I have seen up to 25MB/s over a peice of 1000Mbps fibre.   
>>> Client and Server are both dual P3-1ghz with 1024mb ram. I notice  
>>> the  context switches on the server at this time jump to ~10000/s,  
>>> and on  the client ~40000/s. I imagine this is the source of my  
>>> slowdown, but  I havn't had a chance to look into it.
>>>
>>> I'd be interested if anyone else has the same level of context   
>>> switches going on.
>>>
>>> This is while doing a large sequential write operation (dd   
>>> if=/dev/zero of=/afs/alb-nz.esphion.com/public/dd.out bs=256k   
>>> count=4096).
>>>
>>> Michael Robokoff wrote:
>>>
>>>> Does anyone have any Open AFS performance information they can  
>>>> share  with me. I plan on doing a couple benchmarks and I would  
>>>> like to have  some idea of what to expect.