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Steven Pratt | 2 Oct 2003 21:21
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Re: Re: Minutes from 10/1 LSE Call

Andrew Morton wrote:

>Hanna Linder <hannal <at> us.ibm.com> wrote:
>  
>
>>In mainline, once block size is over 32k our throuput actually drops off.
>>It levels off around 128k but at a greater cpu utilization.
>>Dont really understand why that is. 
>>    
>>
>
>Probably thrashing the CPU's L1 cache.
>  
>
>>In mm tree, maintains throughput for all block size but the cpu utilzation
>>keeps going up to do the same throughput.  Readprofile shows the extra time 
>>is being spent in copy_to_user (in mm tree). Backing out readahead patch 
>>reduces cpu by 10% for all block sizes but still shows the spike. So that
>>isnt the main problem.
>>    
>>
>
>If you have a loop like:
>
>	char *buf;
>
>	for (lots) {
>		read(fd, buf, size);
>	}
>
(Continue reading)

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Andrew Morton | 2 Oct 2003 21:36

Re: Re: Minutes from 10/1 LSE Call

Steven Pratt <slpratt <at> austin.ibm.com> wrote:
>
>  Sure, but why do I only see this is the mm tree, and not the mainline 
>  tree.

Please send a full description of how to reproduce it and I'll take a look.

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Steven Pratt | 3 Oct 2003 21:33
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Re: Re: Minutes from 10/1 LSE Call


Andrew Morton wrote:

>Steven Pratt <slpratt <at> austin.ibm.com> wrote:
>  
>
>> Sure, but why do I only see this is the mm tree, and not the mainline 
>> tree.
>>    
>>
>
>Please send a full description of how to reproduce it and I'll take a look.
>
>  
>
Get the latest rawread from 
http://www-124.ibm.com/developerworks/opensource/linuxperf/rawread/rawread.html

mkfs devices and mount on /mnt/mntN  where N is increasing index.  
Create file 'foo'  in each filesystem of size 1GB (for this example).  
Unmount and remount the partitions/devices to flush the cache.  
Filesystems are also umounted and re-mounted between each test run.

The following rawread commands will run the tests for block sizes 
ranging from 1k-512k.  The "-d 1" parameters assumes that you mounted 
starting at /mnt/mnt1  and the "-m2 -p16" say to run 8 threads on each 
of 2 devices /mnt/mnt1 and /mnt/mnt2.

rawread -m 2 -p 16 -d 6 -n 20480 -f -c -t 0 -s 1024
(Continue reading)

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Andrew Morton | 3 Oct 2003 22:13

Re: Re: Minutes from 10/1 LSE Call

Steven Pratt <slpratt <at> austin.ibm.com> wrote:
>
> Get the latest rawread from 
> http://www-124.ibm.com/developerworks/opensource/linuxperf/rawread/rawread.html

Sigh, I was afraid of that.  My previous outings with rawread have not been
happy.  Maybe your detailed descriptions will help.

> But as long as I have your attention, there is one other thing about 
> these runs which bothers me, which is that the mm tree is doing horribly 
> on 1k and 2k block sizes.  I looks like readahead is not functioning 
> properly for these requst sizes.

There are several problems with readahead under specific circumstances.  I
need to have a session with it.

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