linux-kernel - RE: [PATCH 3/9] [SCSI] mvsas: Add driver version and interrupt coalescing to device attributes in sysfs

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Date:	Mon, 30 May 2011 04:22:40 -0700
From:	Xiangliang Yu <yuxiangl@...vell.com>
To:	Greg KH <greg@...ah.com>
CC:	"James.Bottomley@...e.de" <James.Bottomley@...e.de>,
	"jslaby@...e.cz" <jslaby@...e.cz>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jacky Feng <jfeng@...vell.com>
Subject: RE: [PATCH 3/9] [SCSI] mvsas: Add driver version and interrupt
 coalescing to device 	attributes in sysfs


>> >> +What:          /sys/devices/pci/<devices>/<dev>/host/scsi_host/host/interrupt_coalescing
>> >> +Date:          May 2011
>> >> +Kernel Version:        2.6.39
>> 
>> >2.6.39 was released already, is this file in that release?
>> Yes.

>How, doesn't your patch below implement that option?  How can it already
>be in the .39 kernel?
oh, that is my fault, I read the README and find out the misunderstanding of Kernel
Version. How about 2.6.40?

>> >> +Contact:       yuxiangl@...vell.com
>> >> +Description:   Determines the maximum time the 88SE94XX waits after the occurrence of a
>> >> +               Command Done before generating an interrupt.The maximum number of the 
>> >> +               variable is less than 0x10000.
>> 
>> >Why would a user, or anyone else, ever want to be able to change this?
>> Because different platform can get better performance by setting different value

>Then you need to document _how_ to do this tuning, and why someone would
>want to, and lots of other stuff.  Don't just blindly let userspace
>change a value that they know nothing about.
How about this HOW_TO: you can get a better I/O throughput by decreasing the value of Interrupt coalescing, or you can get a better CPU think time if you increase the value. 

>> >Why wouldn't this just be something that the driver handles
> >>automagically so the user never has to worry about it at all?
> >>As for now, driver can't do it. The value need to be test, and get the best.
>Why don't you test it and set it to the proper value now?  What would
>change in a user's system that require this to be changed?  Size of the
>machine?  Number of disks?  Something else?
>It really should be automatic, people do not ever want to have to
>manually tune their machines anymore they should be smart enough to
>determine the load on them and make the changes without any user needing
>to do it for them.
The default value is best for normal situation, but sometime user need to tune
The value. For example, the system has more than 16 SATA SSD disks, CPU need more time to schedule other jobs while running I/O, but the user want to do lots of other jobs at the same time, so, the user can write a bigger number to the sysfs, the CPU can execute other jobs more quickly. This is a balance
between CPU think time and I/O throughput. But the value is depend on the system environment, like as: disk number, what kind of platform, etc. Actually, the most important reason for tuning is what the user want, I/O throughput or think time.
By default, the value don't need to be changed.
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