> Felix Oxley wrote on Friday, October 28, 2005 5:29 PM
>> Something went horribly wrong with this test between 2.6.13 and
>> 2.6.13-git2 (it has never recovered):
>>
>> System: 4P Itanium
>> Test:Result Group 1
>> Metric: VolcanoMark
>> Result: -3% -10%
>> Kernel: 2.6.13 vs 2.6.13-git2
>>
>> Does anybody know the cause of this?
>
> Search the archive, it was discussed here:
> http://marc.theaimsgroup.com/?l=linux-ia64&m=112683124124723&w=2
>
>
> It is not because of changes in 2.6.13-git2. It would've shown on
> 2.6.13-rc1 when default hz rate was switched to 250. I happened to
> audit the system at that time and made the hz switch (from 1000 to
> 250 and the problem showed up.
>
> More discussion here:
> http://marc.theaimsgroup.com/?l=linux-kernel&m=112854723926854&w=2
>
>
> - Ken
For 4P Itanium, it turns out that Volanaomark is operating
suboptimally with system idle at 55% for the region of operations
where regression occurs for Hz changes. Volanomark server broadcasts
short message (each only ~40 bytes) from each client to the other
clients in the chatroom (20 clients/chatroom in test). However each
message consumes a sk_buff taking up 1 page (16K on Itanium) of
memory as the message is sent immediately without coalescing with
other messages. We hit our default write buffer size limit
tcp_wmem_max (128K) quickly with 8 outstanding packets. The server
stalls waiting for acknowledgement. Due to TCP's delayed
acknowledgement, the server do not get ack immediately to continue.
By either increasing the TCP write buffer size or patching the kernel
to send TCP ACK without delay, we can reduce the system idle to 0%
and Volanomark performance do not show regression in this case when
Hz rate changes.
- Tim
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