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Re: High %user CPU in XFS

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: High %user CPU in XFS
From: Subranshu Patel <spatel.ml@xxxxxxxxx>
Date: Mon, 18 Feb 2013 19:47:34 +0530
Cc: xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
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In-reply-to: <20130217212536.GY26694@dastard>
References: <CAEUQcehy1fy5-W-TA--2emBgL3y9JskqRPCZf1ED5JDbPHqKTQ@xxxxxxxxxxxxxx> <20130217212536.GY26694@dastard>
> Page faults, CPU cache misses. XFS has a significantly large icache
> and dcache footprint than ext4, so those are the likely causes

I have a query. Will page fault, cache misses etc will increase the
%user CPU utilization or %system CPU utilization. I thought it will
affect the %system and not %user CPU usage?

%user - Percentage of CPU utilization that occurred while executing at
the user level (application).
%system - Percentage of CPU utilization that occurred while executing
at the system level (kernel).

On Mon, Feb 18, 2013 at 2:55 AM, Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> 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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