Re: [PATCH 12/22] elevate write count files are open()ed

Dave Hansen
2007-02-14T00:17:45Z

On Tue, 2007-02-13 at 09:58 -0800, Andrew Morton wrote:
> > On Tue, 13 Feb 2007 08:58:16 -0800 Dave Hansen <hansendc@us.ibm.com> wrote:
> > > yipes. A new mount-wide spin_lock/unlock for each for-writing open() and close().
> > > Can we have a microbenchmark on this please?
> >
> > Yeah, I'll schedule some dbench time on a NUMA machine.
>
> dbench doesn't do open() a lot. To assess the worst-case we'd need one
> process per cpu camping in an open/close loop.

This is definitely a worst-case scenario. A 32-way x86_64 NUMA machine
(with a pretty crappy interconnect) with a process-per-cpu all beating
on the same filesystem.

no patch:
real: 30.111s
user: 0.031s
sys: 2.685s

r/o bind mount patch:
real: 48.359s
user: 0.146s
sys: 47.984s

It definitely makes a huge difference in system time, although not a
fatal one. Christoph, what do you think? Back to caching the
superblock flag in the mount?

#!/bin/sh
# go.sh
name=`uname -r`
grep -q /mnt/ram /proc/mounts || mount -t ramfs ram /mnt/ram;
make openbench;
nr_cpus=`cat /proc/cpuinfo | grep -c ^processor`
for ((run=0;run<5;run++)); do
dir=$name.run.$run;
mkdir -p $dir;
for ((i=0;i<nr_cpus;i++)); do
{ time taskset -c $i ./openbench $((1<<16)) & } \
> $dir/openbench.time.$i 2>&1
done;
wait
echo run $run done
done

// openbench.c
#include <stdio.h>

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

void main(int argc, char **argv)
{
pid_t pid = getpid();
char buf[100];
int ret;
int fd;
int i;
int loops = atoi(argv[1]);
sprintf(&buf[0], "/mnt/ram/openbench.%d", pid);

for (i=0; i< loops; i++) {
fd = open(&buf[0], O_WRONLY|O_CREAT);
if (fd < 0) {
perror("open error");
exit(fd);
}
write(fd, "foo");
close(fd);
}
ret = unlink(&buf[0]);
if (ret)
perror("unlink error");
}

-- Dave