Hi, On Fri, 2012-06-01 at 14:14 +0200, Eric Dumazet wrote: > 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 > > >
Why would it return -ENOMEM? It works for me, at worst it will fall back to a single page buffer size unless we are really stuck for memory, and in that case, all bets are off, Steve.
