Erez Zilber wrote:
>> Erez Zilber wrote:
>>> Mike,
>>>
>>> I'm still investigating the performance problems in open-iscsi over iSER, 
>>> and I think that I see the cause of the problem:
>>>
>>> I've added an outstanding commands counter. It's incremented when 
>>> iscsi_queuecommand is called and decremented when iscsi_complete_command is 
>>> called. After decrementing the counter, I save it's value in some global 
>>> array. I print this array when iscsi_session_teardown is called.
>>>
>> >From what I see, in ~25% of command completions, the command queue is empty 
>> >(i.e. outstanding_cmds = 0). This can explain the low throughput that I see 
>> >with iSER.
>>> What could be the cause for that? Is there anything not well configured in 
>>> the scsi_host_template that we use? Anything else?
>>>
>> I saw this when using all io schedulers except noop. When I set the
>> queue to use noop, The iscsi queue was full (host_busy was greater than
>> 1) most of the time and thoughout was higher. I was going by just checking
>>
>> /sys/class/scsi_host/hostX/host_busy
> 
>  
> 
> I'm not familiar with "io schdulers". What's that? You're not talking about 
> benchmraks, right?
> 


No. Sorry, I misspelled it :) io schedulers - maybe you call them 
elevators or something else. The kernel code that determines when to

(write to this to switch schedulers and read it see the ones available)
/sys/block/sda/queue/scheduler
(dir with lots of settings)
/sys/block/sda/queue/iosched

Here is some basic info
http://www.redhat.com/magazine/008jun05/features/schedulers/

There is some more info with the kernel source here: Documentation/block

And then there is the code in the kernel source block dir.

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