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SubjectRe: seq_file: Use larger buffer to reduce time traversing lists
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On Fri, 2012-06-01 at 14:14 +0100, Steven Whitehouse wrote:

> Here it is (with the patch):
>
> [root@chywoon mnt]# time dd if=/sys/kernel/debug/gfs2/unity\:myfs/glocks
> of=/dev/null bs=4k
> 0+5726 records in
> 0+5726 records out
> 23107575 bytes (23 MB) copied, 82.3082 s, 281 kB/s
>
> real 1m22.311s
> user 0m0.013s
> sys 1m22.231s
>
> So thats slow, as promised :-)
>
> > I can't reproduce this slow behavior you have, using /proc/net seq
> > files.
> >
> > Isn't it a problem with this particular file ?
> >
> Well, yes and no. The problem would affect any file with lots of records
> in it, but there may not be many with that number of records. Do any of
> your net files have numbers of entries in the region of hundreds of
> thousands or more?
>
> > Does it want to output a single record ( m->op->show(m, p) ) much larger
> > than 4KB ?
> >
> No. That appears to work ok, so far as I can tell, anyway. What we have
> are lots of relatively short records. Here is an example of a few lines.
> Each line starting G: is a new record, so this is 5 calls to ->show():
>
> G: s:SH n:5/1da5e f:Iqob t:SH d:EX/0 a:0 v:0 r:2 m:200
> H: s:SH f:EH e:0 p:6577 [(ended)] gfs2_inode_lookup+0x116/0x2d0 [gfs2]
> G: s:SH n:2/a852 f:IqLob t:SH d:EX/0 a:0 v:0 r:2 m:200
> I: n:9712/43090 t:8 f:0x00 d:0x00000000 s:0
> G: s:SH n:2/8bcd f:IqLob t:SH d:EX/0 a:0 v:0 r:2 m:200
> I: n:2584/35789 t:8 f:0x00 d:0x00000000 s:0
> G: s:SH n:2/1eea7 f:IqLob t:SH d:EX/0 a:0 v:0 r:2 m:200
> I: n:58968/126631 t:8 f:0x00 d:0x00000000 s:0
> G: s:SH n:2/12fbd f:IqLob t:SH d:EX/0 a:0 v:0 r:2 m:200
> I: n:11120/77757 t:8 f:0x00 d:0x00000000 s:0
>
>
> The key here is that we have a lot of them. My example using just over
> 400k records is in fact a fairly modest example - it is not unusual to
> see millions of records in this file. We use it for debug purposes only,
> and this patch was prompted by people reporting it taking a very long
> time to dump the file.
>
> The issue is not the time taken to create each record, or to copy the
> data, but the time taken each time we have to find our place again in
> the list of glocks (actually a hash table, but same thing applies as we
> traverse it as a set of lists)
>
> I don't think there is really much we can easily do in the case of
> readers requesting small reads of the file. At least we can make it much
> more efficient when they request larger reads though,

Issue is your seq_file provider has O(N^2) behavior

We used to have same issues in network land, and we fixed this some time
ago, and we only use 4KB as seq_file buffer, not a huge one.

Check commit a8b690f98baf9fb1 ( tcp: Fix slowness in
read /proc/net/tcp ) for an example







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