linux-kernel - Re: Thinking outside the box on file systems

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Message-Id: <DDABDE40-EDA1-421A-A3D9-2CE48802A125@mac.com>
Date:	Sat, 18 Aug 2007 01:48:02 -0400
From:	Kyle Moffett <mrmacman_g4@....com>
To:	Phillip Susi <psusi@....rr.com>
Cc:	Valdis.Kletnieks@...edu, Michael Tharp <gxti@...tiallystapled.com>,
	alan <alan@...eserver.org>, Marc Perkel <mperkel@...oo.com>,
	LKML Kernel <linux-kernel@...r.kernel.org>,
	Lennart Sorensen <lsorense@...lub.uwaterloo.ca>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: Thinking outside the box on file systems

On Aug 17, 2007, at 15:01:48, Phillip Susi wrote:
> Valdis.Kletnieks@...edu wrote:
>> It will become even *more* of a "not that common" if the lock will  
>> block moves and ACL changes *across the filesystem* for  
>> potentially *minutes* at a time.
>
> It will not take anywhere NEAR minutes at a time to update the in  
> memory dentries, more like 50ms.

One last comment:

50ms to update in-memory dentries would be FRIGGING TERRIBLE!!!   
Using Perl, an interpreted language, the following script takes 3.39s  
to run on one of my lower-end systems:

for (0 .. 10000) {
	mkdir "a-$_";
	mkdir "b-$_";
	rename "a-$_", "b-$_";
}

It's not even deleting things afterwards so it's populating a  
directory with ten thousand entries.  We can easily calculate  
10,000/3.39 = 2,949 entries per second, or 0.339 milliseconds per entry.

When I change it to rmdir things instead, the runtime goes down to  
2.89s == 3460 entries/sec == 0.289 milliseconds per entry.

If such a scheme even increases the overhead of a directory rename by  
a hundredth of a millisecond on that box it would easily be a 2-3%  
performance hit.  Given that people tend to kill for 1% performance  
boosts, that's not likely to be a good idea.

Cheers,
Kyle Moffett

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