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

[Paul Mackerras]

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.

> 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).

Paul.
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