Messages in this thread | | | Subject | Re: seq_file: Use larger buffer to reduce time traversing lists | From | Eric Dumazet <> | Date | Fri, 01 Jun 2012 14:14:30 +0200 |
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On Fri, 2012-06-01 at 14:10 +0200, Eric Dumazet wrote: > On Fri, 2012-06-01 at 11:39 +0100, Steven Whitehouse wrote: > > I've just been taking a look at the seq_read() code, since we've noticed > > that dumping files with large numbers of records can take some > > considerable time. This is due to seq_read() using a buffer which, at > > most is a single page in size, and that it has to find its place again > > on every call to seq_read(). That makes it rather inefficient. > > > > As an example, I created a GFS2 filesystem with 100k inodes in it, and > > then ran ls -l to get a decent number of cached inodes. This result in > > there being approx 400k lines in the debugfs file containing GFS2's > > glocks. I then timed how long it takes to read this file: > > > > [root@chywoon mnt]# time dd if=/sys/kernel/debug/gfs2/unity\:myfs/glocks > > of=/dev/null bs=1M > > 0+5769 records in > > 0+5769 records out > > 23273958 bytes (23 MB) copied, 63.3681 s, 367 kB/s > > What time do you get if you do > > time dd if=/sys/kernel/debug/gfs2/unity\:myfs/glocks of=/dev/null bs=4k > > This patch seems the wrong way to me. > > seq_read(size = 1MB) should perform many copy_to_user() calls instead of a single one. > > Instead of doing kmalloc(m->size <<= 1, GFP_KERNEL) each time we overflow the buffer, > we should flush its content to user space. > >
by the way, is the following command even working ?
time dd if=/sys/kernel/debug/gfs2/unity\:myfs/glocks of=/dev/null bs=16M
I guess not, it probably returns -ENOMEM
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