linux-kernel - Re: [PATCH 0/3] i_ino uniqueness: alternate approach -- hash the inodes

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Date:	Wed, 24 Jan 2007 16:21:44 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Jeff Layton <jlayton@...hat.com>
Cc:	Andrew Morton <akpm@...l.org>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] i_ino uniqueness: alternate approach -- hash the inodes

On Wednesday 24 January 2007 15:22, Jeff Layton wrote:
> Andrew Morton wrote:
> > What is the additional overhead, expressed in relative terms?  ie: as a
> > percentage?
>
> Short answer: ~3-4% in a not very scientific test.
>
> Long answer: I timed 3 different runs of a program that created and then
> closed a pipe 10 million times on a patched and unpatched kernel. I then
> added up the "system" times for each and divided them:

Do you mean this program ?

int count, pfd[2];
for (count = 0 ; count < 10000000 ; count++) {
	pipe(pfd);
	close(pfd[0]);
	close(pfd[1]);
}

The problem is you wont see the overhead of insert/delete the inode in a 
global tree, since you keep hot caches.

To have a better estimate of the overhead, I suggest you try to use more 
active pipes like :

#include <unistd.h>
#define SIZE 16384
int fds[SIZE];

int main(int argc, char *argv[])
{
        unsigned int i , count ;

        for (i = 0 ; i < SIZE ; i += 2)
                pipe(fds + i);
        i = 0;
        for (count = 0 ; count < 10000000 ; count++) {
                close(fds[i]);
                close(fds[i + 1]);
                pipe(fds + i);
                i = (i + 2) % SIZE;
        }
        return 0;
}


# ulimit -n 20000
# time ./pipebench

Eric
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