[OpenAFS-devel] good and bad performance of memory cache

[Edward Moy]

On Thursday, February 6, 2003, at 01:27  PM, Lyle Seaman wrote:

>> On the server side, memcache resulted in about 1.4 million input and
>> 0.92 million output packets, while diskcache was 0.80 million input  
>> and
>> 0.90 million output.  For the fileserver process, 1:46 cpu minutes for
>> memcache and 1:03 cpu minutes for diskcache.  The 1 minute load  
>> average
>> never exceeded 0.45 in my samples, though the 5 minute load average
>> peaked at 0.28 for memcache, 0.21 for diskcache.
>
> neat.  So, what do you think is happening?
>
>> NFS ... resulted in 0.87 million input and 0.62 million output  
>> packets.
>
> That would be as seen from the server, right?

Yes, from the server side.

The results of iozone itself might provides some clues (I split the  
results into two parts; units are Kbytes/sec):

                                                             random   
random
               KB  reclen   write rewrite    read    reread    read    
write
memcache  131072      64    4223    1262     2328     2323  206218     
3810
diskcache 131072      64    4114    1821     5849     5847  157343     
3599

            bkwd  record  stride
            read rewrite    read   fwrite frewrite   fread  freread
memcache   5260  211525    2891     3603     1267    2288     2284
diskcache 11370   87494    7071     4146     1816    5865     5852

Notice that except for random reads (where the memory cache is showing  
its advantage), all other memcache reads are less than half as much the  
corresponding value for diskcache.

Also, the rewrite speed is a lot slower than the first write.  This  
also happens for file sizes smaller than the cache size, at the smaller  
record sizes, but as the record size increase, the rewrite eventually  
becomes larger than the first write.  But once the file size is larger  
than the cache size, the rewrite speed tends to level off at a low  
value, while the first write value is still many times higher.

And read values for memcache are also less than memcache write speeds  
(except the rewrite value is worse than reads).
------------------------------------------------------------------------ 
--
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.)