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From: Nick Piggin <nickpiggin@yahoo.com.au>
To: Paul Mackerras <paulus@samba.org>
Subject: Re: [perfmon] Re: [perfmon2] perfmon2 merge news
Date: Wed, 14 Nov 2007 21:17:55 +1100
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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
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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.
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