High %user CPU in XFS

[Dave Chinner]

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 at fromorbit.com