Re: NFS client latencies

Ingo Molnar
2005-03-31T07:35:13Z

* Andrew Morton <akpm@osdl.org> wrote:

> 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.

The list-based approach's best-case are large continuous append writes.
No sorting overhead, and light data structures.

i'd say this workload should be not that bad under the radix tree either
- the gang lookup stuffs a nice vector of 16 pages into an array.

we definitely can say nothing based on the observation that a _single_
page took 1.9 msecs in Lee's previous measurement, while 7700 pages now
take 6 msecs to process.

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

the generic VM/pagecache has proven that the radix tree wins hands down
for alot more workloads than the worst-case.

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

that was my other point in a previous mail: most write benchmarks do
continuous append writes, and CPU overhead easily gets lost in network
latency.

Also, considering that a good portion of the NFS client's code is still
running under the BKL one would assume if the BKL hurts performance it
would have been changed by now? ;-)

Ingo