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Re: High %user CPU in XFS

To: Subranshu Patel <spatel.ml@xxxxxxxxx>
Subject: Re: High %user CPU in XFS
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Mon, 18 Feb 2013 08:25:36 +1100
Cc: xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <CAEUQcehy1fy5-W-TA--2emBgL3y9JskqRPCZf1ED5JDbPHqKTQ@xxxxxxxxxxxxxx>
References: <CAEUQcehy1fy5-W-TA--2emBgL3y9JskqRPCZf1ED5JDbPHqKTQ@xxxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Sun, Feb 17, 2013 at 09:20:46PM +0530, Subranshu Patel wrote:
>  I created 2 filesystem on my system (RHEL 6.3, kernel 2.6.32) - XFS
> and EXT4 and mounted them.
> 
> On both the filesystem I executed a program which performed the following:
> - Create large number of directories
> - Removing all the directories
> 
> During the execution of the program, I monitored the %user and %system
> CPU usage (using sar command)
> 
> For EXT4 the %user was 1% and %system was 21%
> For XFS the %user was 20% and %system was 52%
> 
> But I am not able to understand %user consumption, since %user reports
> the CPU usage of the user level (application), shouldn't it be the
> same for both EXT4 and XFS?

Perf is your friend.

> What are the factors that decide the %user component keeping in mind
> that only the filesystem is different?

Page faults, CPU cache misses. XFS has a significantly large icache
and dcache footprint than ext4, so those are the likely causes.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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