Re: ls command is slow..... (reiserfs, VM)?

[R.E.Wolff@BitWizard.nl (Rogier Wolff)]

Nikita Danilov wrote:
> Rogier Wolff writes:
>  > 
>  > Hi,
>  > 
>  > the "ls" command is horribly slow on my system....
>  > 
>  > I'm doing some stuff with large files. From the old days when files
>  > couldn't be larger than 2G, I'm manipulating 1G files.
>  > 
>  > There is currently a program runnning that will make a file sparse if
>  > it finds only zeroes in a block. It's reading between 20 and 30Mb per
>  > second off the disks, and currently finding only zeros, so there is no
>  > writing going on.
> 
> Reiserfs puts on-disk inode (stat-data) near file "body" which is an
> array of pointers to actual blocks of the file. This optimizes the case
> of short files, because inode and file "body" can be read in one io. But
> when there are many large files in the same directory, this results in
> inodes of the files from the same directory being far from each other on
> the disk, making readdir() or sequential stat() slower. Reiser4 uses
> (will use, that is) different allocation policy that tries to address
> this.

OK. so the layout on the disk is non-optimal. 

But I'd expect performance on the order of: 

	
times 
in ms

0	my first stat finishes
0.1	the other program has already issued a 1Mb read from the 
	other part of the disk. 
	my ls issues a second stat. 
10 	seek finishes
18	rotational latency. 
58	read (1Mb) finishes (25Mb per second -> 40ms /Mb). 
68	seek to "my ls" finishes. 
76	rotational latency. 
76.1	my stat has transferred its 1k, in the intervening time the
	other program has already issued a new read for 1Mb of data.
86	seek to other program's data finishes... 

So I'd expect something like 10-14 stats per second. While in fact my
trace showed 3 to 5 stats per second. Still at 50 files in a directory
waiting around 4-5 seconds to see results from an "ls" is kind of
annoyingly slow, and I would've reported it as well.

Maybe the stat requires 2 to 3 IOs to complete? That would explain the
difference.

So the question becomes: Why does a stat require 2 or 3 IOs? OR why is
too much independent stuff scheduled inbetween that one IO takes
200-300 ms?...


		Roger. 

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