linux-kernel - Re: [PATCH] [RFC] Adjust queue unplugging and congestion limits

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Message-Id: <20070624123350.edf433bc.akpm@linux-foundation.org>
Date:	Sun, 24 Jun 2007 12:33:50 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Patrick Mau <mau@...ar.ping.de>
Cc:	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] [RFC] Adjust queue unplugging and congestion limits

On Sun, 24 Jun 2007 19:27:23 +0200 Patrick Mau <mau@...ar.ping.de> wrote:

> Good afternoon
> 
> Following is a small patch to "ll_rw_block" I played around with.
> I started using "blktrace" to analyse the performance of my dated
> LVM / MD / SCSI setup. It's really a nice tool, by the way.
> 
> The changes are:
> 
> * Change the "q->unplug_thresh" to be halve of "q->nr_requets".
>   This dynamically delays unplugging. It was hardcoded to 4 and
>   makes a difference in performace because of higher merge counts.
> 
> * Change the unplug timeout to 10 milliseconds.
>   This is of course questionable, but I don't know why 3 was
>   choosen in the first place. I just played with it.
> 
> * Set the congestion on/off limits further apart.
> 
> * Cleanup the congestion checks (no "+1" anymore, more readable).
> 
> * Update "q->unplug_thresh" whenever "q->nr_requests" is updated
>   through sysfs.

I don't think anyone has played with these settings since they first went
in and yes, there may be some benefits available here.

> I did various artifical benchmarksi, mostly tar, dd and cp (also to NFS).
> Comments are welcome.

umm, what was the result of this benchmarking?  That is rather important
information ;)

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