amrd disk performance drop after running under high load

Kris Kennaway kris at
Tue Oct 16 11:30:57 PDT 2007

Alexey Popov wrote:
> Hi.
> Kris Kennaway wrote:
>>>>> After some time of running under high load disk performance become 
>>>>> expremely poor. At that periods 'systat -vm 1' shows something like
>>>>> this:
>>>> What does "high load" mean?  You need to explain the system workload 
>>>> more.
>>> This web service is similiar to YouTube. This server is video store. I
>>> have around 200G of *.flv (flash video) files on the server.
>>> I run lighttpd as a web server. Disk load is usually around 50%, network
>>> output 100Mbit/s, 100 simultaneous connections. CPU is mostly idle.
>>> As you can see it is a trivial service - sending files to network via 
>>> HTTP.
>> Does lighttpd actually use HTTP accept filters?
> Don't know how to make sure, but is seems to run appropriate setsockopt 
> (truss output):
> setsockopt(0x4,0xffff,0x1000,0x7fffffffe620,0x100) = 0 (0x0)


>> Are you using ipfilter and ipfw?  You are paying a performance penalty 
>> for having them.
> I'm using ipfw and one of the first rules is to pass all TCP 
> established. ipfilter is not used on this server, but it is present in 
> kernel as it can be used on other servers. I have 95% CPU idle, so I 
> think packet filters does not produce significant load on this server.

Well, it was not your most serious issue, but from your profiling trace 
it is definitely burning cycles with every packet processed.

>> You might try increasing BUCKET_MAX in sys/vm/uma_core.c.  I don't 
>> really understand the code here, but you seem to be hitting a 
>> threshold behaviour where you are constantly running out of space in 
>> the per CPU caches.
> Thanks, I'll try this.
>> This can happen if your workload is unbalanced between the CPUs and 
>> you are always allocating on one but freeing on another, but I 
>> wouldn't expect it should happen on your workload.  Maybe it can also 
>> happen if your turnover is high enough.  
> This is very unlikely, because I have 5 another video storage servers of 
> the same hardware and software configurations and they feel good.

Clearly something is different about them, though.  If you can 
characterize exactly what that is then it will help.

> On the other side, all other servers were put in production before or 
> after problematic servers and were filled with content in the other ways 
> and therefore they could have slightly differerent load pattern.
> Totally I faced this bug three times:
> 1. The first time there was AFAIR 5.4-RELEASE on DELL 2850 with the same 
> configuration as now. It was mp3 store and I used thttpd as HTTP server 
> to serve mp3's. That time the problems were not so frequent and also it 
> took too long to get back to normal operation so we had to reboot 
> servers once a week or so.
> The problems began when we moved to new hardware - Dell 2850. That time 
> we suspected amrd driver and had no time to dig in, bacause all the 
> servers of the project were problematic. Installing Linux helped.
> 2. The second time it was server for static files of the very popular 
> blog. The http server was nginx and disk contented puctures, mp3's and 
> videos. It was Dell 1850 2x146 SCSI mirror. Linux also solved the problem.
> 3. The problem we see now.
> At first glance one can say that problem is in Dell's x850 series or 
> amr(4), but we run this hardware on many other projects and they work 
> well. Also Linux on them works.

OK but there is no evidence in what you posted so far that amr is 
involved in any way.  There is convincing evidence that it is the mbuf 

> And few hours ago I received feed back from Andrzej Tobola, he has the 
> same problem on FreeBSD 7 with Promise ATA software mirror:

Well, he didnt provide any evidence yet that it is the same problem, so 
let's not become confused by feelings :)

> 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.


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