Re: [perfmon] Re: [perfmon2] perfmon2 merge news

[Nick Piggin <nickpiggin@yahoo.com.au>]

On Thursday 15 November 2007 08:30, Paul Mackerras wrote:
> Nick Piggin writes:
> > What I really mean is a readv-like syscall, but one that also
> > vectorises the file offset. Maybe this is useful enough as a generic
> > syscall that also helps Paul's example...
>
> I've sometimes thought it would be useful to have a "transaction"
> system call that is like a write + read combined into one:
>
> 	int transaction(int fd, char *req, size_t req_nb,
> 			char *reply, size_t reply_nb);
>
> as a way to provide a general request/reply interface for special
> files.

Maybe not a bad idea, though I'm not the one to ask about taste ;)
In this case, it is enough for your requests to be a set of scalars
(eg. file offsets), so it _could_ be handled with vectorised offsets...

But in general, for special files, I guess the response is usually
some structured data (that is not visible at the syscall layer).
So I don't see a big problem to have a similarly arbitrarily
structured request.


> > Of course, I guess this all depends on whether the atomicity is an
> > important requirement. If not, you can obviously just do it with
> > multiple read syscalls...
>
> That would take N system calls instead of one, which could have a
> performance impact if you need to read the counters frequently (which
> I believe you do in some performance monitoring situations).

That's true too.