linux-kernel - Re: [dm-devel] Questions about the function, queue

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Message-ID: <20150105091707.GA11818@debian>
Date:	Mon, 5 Jan 2015 09:17:08 +0000
From:	Joe Thornber <thornber@...hat.com>
To:	device-mapper development <dm-devel@...hat.com>
Cc:	agk@...hat.com, linux-raid@...r.kernel.org,
	linux-kernel@...r.kernel.org, snitzer@...hat.com
Subject: Re: [dm-devel] Questions about the function, queue_empty in
 dm-cache-policy-mq.c

On Sun, Jan 04, 2015 at 10:24:16PM -0500, nick wrote:
> Greetings All,
> I have a few questions about the function I stated in the message for this email.
> 1. What is the issues with the CPU usage for this function?

At the moment it can perform up to NR_QUEUE_LEVELS (16) list_empty()
calls.  Which I consider too much work for a function that is called
frequently.

> 2. What hardware is this on ?

Irrelevant.

> 3. How scalable is this function in terms of NR_QUEUE_LEVELS and how many does it need to handle?

Linear.  NR_QUEUE_LEVELS is a compile time constant currently set to
16, and unlikely to change.

- Joe
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