linux-kernel - Re: Understanding I/O behaviour

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Date:	Thu, 30 Aug 2007 03:50:05 -0700 (PDT)
From:	Martin Knoblauch <spamtrap@...bisoft.de>
To:	Jens Axboe <jens.axboe@...cle.com>,
	Martin Knoblauch <spamtrap@...bisoft.de>
Cc:	linux-kernel@...r.kernel.org,
	Peter zijlstra <a.p.zijlstra@...llo.nl>, mingo@...hat.com
Subject: Re: Understanding I/O behaviour - next try


--- Jens Axboe <jens.axboe@...cle.com> wrote:

> 
> Try limiting the queue depth on the cciss device, some of those are
> notoriously bad at starving commands. Something like the below hack,
> see
> if it makes a difference (and please verify in dmesg that it prints
> the
> message about limiting depth!):
> 
> diff --git a/drivers/block/cciss.c b/drivers/block/cciss.c
> index 084358a..257e1c3 100644
> --- a/drivers/block/cciss.c
> +++ b/drivers/block/cciss.c
> @@ -2992,7 +2992,12 @@ static int cciss_pci_init(ctlr_info_t *c,
> struct pci_dev *pdev)
>  		if (board_id == products[i].board_id) {
>  			c->product_name = products[i].product_name;
>  			c->access = *(products[i].access);
> +#if 0
>  			c->nr_cmds = products[i].nr_cmds;
> +#else
> +			c->nr_cmds = 2;
> +			printk("cciss: limited max commands to 2\n");
> +#endif
>  			break;
>  		}
>  	}
> 
> -- 
> Jens Axboe
> 
> 
Hi Jens,

 how exactely is the queue depth related to the max # of commands? I
ask, because with the 2.6.22 kernel the "maximum queue depth since
init" seems to be never higher than 16, even with much higher
outstanding commands. On a 2.6.19 kernel, maximum queue depth is much
higher, just a bit below "max # of commands since init".

[2.6.22]# cat /proc/driver/cciss/cciss0
cciss0: HP Smart Array 6i Controller
Board ID: 0x40910e11
Firmware Version: 2.76
IRQ: 51
Logical drives: 1
Max sectors: 2048
Current Q depth: 0
Current # commands on controller: 145
Max Q depth since init: 16
Max # commands on controller since init: 204
Max SG entries since init: 31
Sequential access devices: 0

[2.6.19] cat /proc/driver/cciss/cciss0
cciss0: HP Smart Array 6i Controller
Board ID: 0x40910e11
Firmware Version: 2.76
IRQ: 51
Logical drives: 1
Current Q depth: 0
Current # commands on controller: 0
Max Q depth since init: 197
Max # commands on controller since init: 198
Max SG entries since init: 31
Sequential access devices: 0

Cheers
Martin




------------------------------------------------------
Martin Knoblauch
email: k n o b i AT knobisoft DOT de
www:   http://www.knobisoft.de
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