linux-kernel - Re: [perfmon] Re: [perfmon2] perfmon2 merge news

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Message-ID: <18235.26873.437786.651828@cargo.ozlabs.ibm.com>
Date:	Thu, 15 Nov 2007 08:30:33 +1100
From:	Paul Mackerras <paulus@...ba.org>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	David Miller <davem@...emloft.net>, hch@...radead.org,
	akpm@...ux-foundation.org, gregkh@...e.de, mucci@...utk.edu,
	eranian@....hp.com, wcohen@...hat.com, robert.richter@....com,
	linux-kernel@...r.kernel.org, andi@...stfloor.org
Subject: Re: [perfmon] Re: [perfmon2] perfmon2 merge news

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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