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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@samba.org>
To: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: David Miller <davem@davemloft.net>, hch@infradead.org,
       akpm@linux-foundation.org, gregkh@suse.de, mucci@cs.utk.edu,
       eranian@hpl.hp.com, wcohen@redhat.com, robert.richter@amd.com,
       linux-kernel@vger.kernel.org, andi@firstfloor.org
Subject: Re: [perfmon] Re: [perfmon2] perfmon2 merge news
In-Reply-To: <200711141125.18568.nickpiggin@yahoo.com.au>
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	<20071114.035850.202836100.davem@davemloft.net>
	<200711141125.18568.nickpiggin@yahoo.com.au>
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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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