Re: Use of READDIRPLUS on large directories

[NeilBrown <neilb@suse.de>]

On Wed, 16 Mar 2011 10:20:03 -0400 Trond Myklebust
<Trond.Myklebust@netapp.com> wrote:

> On Wed, 2011-03-16 at 10:14 -0400, Bryan Schumaker wrote:
> > I guess I misunderstood what to publish test results for?  I know I included numbers on one of the patches (commit 82f2e5472e2304e531c2fa85e457f4a71070044e, copied below)...  I'll find the numbers you're asking about and post them.
> > 
> > -Bryan
> > 
> > commit 82f2e5472e2304e531c2fa85e457f4a71070044e
> > Author: Bryan Schumaker <bjschuma@netapp.com>
> > Date:   Thu Oct 21 16:33:18 2010 -0400
> > 
> >     NFS: Readdir plus in v4
> >     
> >     By requsting more attributes during a readdir, we can mimic the readdir plus
> >     operation that was in NFSv3.
> >     
> >     To test, I ran the command `ls -lU --color=none` on directories with various
> >     numbers of files.  Without readdir plus, I see this:
> >     
> >     n files |    100    |   1,000   |  10,000   |  100,000  | 1,000,000
> >     --------+-----------+-----------+-----------+-----------+----------
> >     real    | 0m00.153s | 0m00.589s | 0m05.601s | 0m56.691s | 9m59.128s
> >     user    | 0m00.007s | 0m00.007s | 0m00.077s | 0m00.703s | 0m06.800s
> >     sys     | 0m00.010s | 0m00.070s | 0m00.633s | 0m06.423s | 1m10.005s
> >     access  | 3         | 1         | 1         | 4         | 31
> >     getattr | 2         | 1         | 1         | 1         | 1
> >     lookup  | 104       | 1,003     | 10,003    | 100,003   | 1,000,003
> >     readdir | 2         | 16        | 158       | 1,575     | 15,749
> >     total   | 111       | 1,021     | 10,163    | 101,583   | 1,015,784
> >     
> >     With readdir plus enabled, I see this:
> >     
> >     n files |    100    |   1,000   |  10,000   |  100,000  | 1,000,000
> >     --------+-----------+-----------+-----------+-----------+----------
> >     real    | 0m00.115s | 0m00.206s | 0m01.079s | 0m12.521s | 2m07.528s
> >     user    | 0m00.003s | 0m00.003s | 0m00.040s | 0m00.290s | 0m03.296s
> >     sys     | 0m00.007s | 0m00.020s | 0m00.120s | 0m01.357s | 0m17.556s
> >     access  | 3         | 1         | 1         | 1         | 7
> >     getattr | 2         | 1         | 1         | 1         | 1
> >     lookup  | 4         | 3         | 3         | 3         | 3
> >     readdir | 6         | 62        | 630       | 6,300     | 62,993
> >     total   | 15        | 67        | 635       | 6,305     | 63,004
> >     
> >     Readdir plus disabled has about a 16x increase in the number of rpc calls an
> >     is 4 - 5 times slower on large directories.
> 
> Right. Those are the numbers that convinced me...
> 
> 

Lies, Damn Lies, and ......


while these are impressive numbers they only tell half the story.

If a change makes one common operation 4 times faster, and another common
operation 10 times slower, it is a good change?  or even an acceptable change?

(The "10 times" is not a definite statistic - it is a guess based on
 a low-detail report)

So it is obvious that there is sometimes value in using readdirplus,
it is equally obvious that there is sometimes a cost.

Switching the default from "not paying the cost when it is big" to
"always paying the cost" is wrong.


NeilBrown