amrd disk performance drop after running under high load

Kris Kennaway kris at
Wed Oct 31 14:38:22 PDT 2007

Alexey Popov wrote:
> Hi
> Kris Kennaway wrote:
>>>>>>> So I can conclude that FreeBSD has a long standing bug in VM that 
>>>>>>> could be triggered when serving large amount of static data (much 
>>>>>>> bigger than memory size) on high rates. Possibly this only 
>>>>>>> applies to large files like mp3 or video. 
>>>>>> It is possible, we have further work to do to conclude this though.
>>>>> I forgot to mention I have pmc and kgmon profiling for good and bad 
>>>>> times. But I have not enough knowledge to interpret it right and 
>>>>> not sure if it can help.
>>>> pmc would be useful.
>>> pmc profiling attached.
>> OK, the pmc traces do seem to show that it's not a lock contention 
>> issue.  That being the case I don't think the fact that different 
>> servers perform better is directly related. 
> But it was evidence of mbuf lock contention in mutex profiling, wasn't 
> it? As far as I understand, mutex problems can exist without increasing 
> CPU load in pmc stats, right?

No, the lock functions will show up as using a lot of CPU.  I guess the 
lock profiling trace showed high numbers because you ran it for a long time.

>> There is also no evidence of a VM problem.  What your vmstat and pmc 
>> traces show is that your system really isn't doing much work at all, 
>> relatively speaking.
>> There is also still no evidence of a disk problem.  In fact your disk 
>> seems to be almost idle in both cases you provided, only doing between 
>> 1 and 10 operations per second, which is trivial.
> vmstat and network output graphs shows that the problem exists. If it is 
> not a disk or network or VM problem, what else could be wrong?

The vmstat output you provided so far doesn't show anything specific.

>> In the "good" case you are getting a much higher interrupt rate but 
>> with the data you provided I can't tell where from.  You need to run 
>> vmstat -i at regular intervals (e.g. every 10 seconds for a minute) 
>> during the "good" and "bad" times, since it only provides counters and 
>> an average rate over the uptime of the system.
> I'll try this, but AFAIR there was no strangeness with interrupts.
> I believe the reason of high interrupt rate in "good" cases is that 
> server sends much traffic.
>> What there is evidence of is an interrupt aliasing problem between em 
>> and USB:
>> irq16: uhci0                  1464547796       1870
>> irq64: em0                    1463513610       1869
> I tried disabling USB in kernel, this ussie was gone, but the main 
> problem was left. Also I have this issue with interrupt aliasing on many 
> servers without problems.



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