Re: [Lse-tech] lockmeter results comparing 2.4.17, 2.5.3, and 2.5.5

Linus Torvalds (torvalds@transmeta.com)
Wed, 27 Feb 2002 19:27:18 +0000 (UTC)


In article <67850000.1014834875@flay>,
Martin J. Bligh <Martin.Bligh@us.ibm.com> wrote:
>
>Whilst it's great to see BKL contention going down, this:
>
>0.16% 0.25% 0.7us( 100ms) 252us( 86ms)(0.02%) 6077746 99.8% 0.25% 0% inode_lock
> 0.03% 0.11% 0.6us( 55us) 2.1us( 9.9us)(0.00%) 1322338 99.9% 0.11% 0% __mark_inode_dirty+0x48
> 0.00% 0% 0.7us( 5.9us) 0us 391 100% 0% 0% get_new_inode+0x28
> 0.00% 0.22% 2.5us( 50us) 495us( 28ms)(0.00%) 50397 99.8% 0.22% 0% iget4+0x3c
> 0.03% 0.28% 0.6us( 26us) 30us( 58ms)(0.00%) 1322080 99.7% 0.28% 0% insert_inode_hash+0x44
> 0.04% 0.29% 0.5us( 39us) 332us( 86ms)(0.01%) 2059365 99.7% 0.29% 0% iput+0x68
> 0.03% 0.30% 0.7us( 57us) 422us( 77ms)(0.01%) 1323036 99.7% 0.30% 0% new_inode+0x1c
> 0.03% 8.3% 63ms( 100ms) 3.8us( 3.8us)(0.00%) 12 91.7% 8.3% 0% prune_icache+0x1c
> 0.00% 0% 1.0us( 5.2us) 0us 34 100% 0% 0% sync_unlocked_inodes+0x10
> 0.00% 0% 1.0us( 2.4us) 0us 93 100% 0% 0% sync_unlocked_inodes+0x110

Ugh. 100ms max holdtime really looks fairly disgusting.

Since that code only does list manipulation, that implies that the
inode_unused list is d*mn long.

>looks a little distressing - the hold times on inode_lock by prune_icache
>look bad in terms of latency (contention is still low, but people are still
>waiting on it for a very long time). Is this a transient thing, or do people
>think this is going to be a problem?

That already _is_ a problem: 0.1 seconds of hickups is nasty (and
obviously while it wasn't called that many times, several other users
actually hit it for almost that long).

It _sounds_ like we may put inodes on the unused list when they really
aren't unused. I don't think you can get the mean time that high
otherwise (63ms _mean_ lock hold time? Do I really read that thing
correctly?).

Actually, looking at "prune_icache()", I think we have two problems:

- inodes may be too often on the unuse-list when they cannot be unused

- thse kinds of inodes tend to cluster at the _head_ of the list, and
we never correct for that. So once we get into that situation, we
stay there.

Al, comments? I note:

- in __iget(), if the inode count goes from 0 to 1, we'll leave it on
the unused list if it is locked (if it is dirty it should already
have been on the dirty list, so that case should be ok)

But I don't really see where there would be any bigger bug..

- is this the time to get rid of the "inode_unused" thing entirely? If
we drop the thing from the dcache, we might as well drop the inode
too.

Ehh..

Linus
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