Re: NFS client latencies

[Andrew Morton <akpm@osdl.org>]

Ingo Molnar <mingo@elte.hu> wrote:
>
> 
>  * Trond Myklebust <trond.myklebust@fys.uio.no> wrote:
> 
>  > > The 7 ms are spent in this loop:
>  >
>  > Which is basically confirming what the guys from Bull already told me, 
>  > namely that the radix tree tag stuff is much less efficient that what 
>  > we've got now. I never saw their patches, though, so I was curious to 
>  > try it for myself.
> 
>  i think the numbers are being misinterpreted. I believe this patch is a 
>  big step forward. The big thing is that nfs_list_add_request() is O(1) 
>  now - while _a single request addition to the sorted list_ triggered the 
>  1+ msec latency in Lee's previous trace.

Well.  The radix-tree approach's best-case is probably quite a lot worse
than the list-based approach's best-case.  It hits a lot more cachelines
and involves a lot more code.

But of course the radix-tree's worst-case will be far better than list's.

And presumably that list-based code rarely hits the worst-case, else it
would have been changed by now.