Re: [Scst-devel] scsi cmd performance

[Vladislav B. <vs...@vl...>]

Ming Zhang wrote:
> On Thu, 2005-11-17 at 21:21 +0300, Vladislav Bolkhovitin wrote:
> 
>>Ming Zhang wrote:
>>
>>>what i mean is when a scsi command received, it is sent to middle layer,
>>>do this unplug, executed by scsi device, respond back to initiator. then
>>
>>Well, why do you think "then"? An initiator doesn't have to wait the 
>>command's completition in order to send the next one.
> 
> 
> for disk, this is not true. but for tape device, it will be true i
> think.

??

>>>next scsi command come in. so scsi cmd is serially executed from
>>>initiator point of view.
>>>
>>>if we have initiator, scst box, scsi storage array, then this scst box
>>>is like a bridge box while serialize all scsi traffic.
>>>
>>>maybe i understand something wrong.
>>
>>Looks like you should check what "plugging" means.
> 
> 
> guess so. could u explain a bit? thanks.
> 
> my understanding is that unplug will release the queue and began to
> execute, while code will not wait i think. so this is different from
> scsi_wait_req().

Seems, that's correct, although scsi_wait_req()/scsi_req_req() is, 
basically, orthogonal to the "plugging".

The code will not wait in any case, even without doing unplugging

> ming
> 
> 
>>>Ming
>>> 
>>>On Thu, 2005-11-17 at 19:57 +0300, Vladislav Bolkhovitin wrote:
>>>
>>>
>>>>Hm, I don't see in any of those 2 places a "synchronousation" point. 
>>>>Both putting scsi cmd in the queue and generic_unplug_device() are 
>>>>asynchronous. So, what is the question?
>>>>
>>>>Vlad
>>>>
>>>>Ming Zhang wrote:
>>>>
>>>>
>>>>>Hi folks
>>>>>
>>>>>I wonder how scst maintain the speed if it act as a bridge like device.
>>>>>In scst_send_to_midlev(), it calls scst_do_send_to_midlev() to put scsi
>>>>>cmd in the queue, then it will call generic_unplug_device() to execute
>>>>>the queue. So will all scsi command sent from initiator side will be
>>>>>executed one by one in a synchronous way? if so, will the speed be
>>>>>greatly reduced?
>>>>>
>>>>>
>>>>>
>>>>>Ming