Yes, there will be additional context switch costs for a user-land
implementation. However, where a disk I/O costs a luxury yacht a context
switch might cost a used bicycle. So I think filesystem designers are in
the position of not worrying about the old bike because it's best to focus
negotiating efforts on the yacht. So I guess the question on our mind was;
is OCaml another luxury yacht?

(With the NFS metadata operations in BDBFS there were synchronous I/O
operations on the path. These will make a context switch insignificant.
Consider the milliseconds required for an I/O.)

To narrow the experiment to isolating the language cost, I eliminated the
synchronous I/O by placing the DB->put()s outside of a transaction, with
no commit. As I'm sure you realised, all of the C and OCaml Native and
Bytecode experiments were run in user-land so all had additional context
switches above a kernel-level implementation. However, given I/O costs in
filesystems, context switch costs are insignificant.

Lex

On Thu, 1 May 2003, Miles Egan wrote:

> On Thu, 2003-05-01 at 10:29, Lex Stein wrote:
> > Hi,
> >
> > A while ago I built an NFS server in OCaml (BDBFS) and the performance
> > stunk. It was 10x slower than the BSD in-kernel NFS server for metadata
> > operations. There was some speculation about what was causing this
> > slowness. It could have been a number of things. So in order for my
> > Advisor to let me continue programming in OCaml, I set out to show that it
> > wasn't due to the choice of OCaml.
>
> Wouldn't you expect any userspace nfs server to be much slower than the
> kernel-based implementation due to the overhead of all the extra
> context-switching?
>
> --
> Miles Egan <miles@caddr.com>
>

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