Date: Sat, 19 Dec 2009 21:32:19 +0100 From: Ulrich =?utf-8?B?U3DDtnJsZWlu?= <uqs@spoerlein.net> To: Harti Brandt <hartmut.brandt@dlr.de> Cc: bde@freebsd.org, freebsd-arch@freebsd.org Subject: Re: network statistics in SMP Message-ID: <20091219203219.GS55913@acme.spoerlein.net> In-Reply-To: <20091219154206.E93919@beagle.kn.op.dlr.de> References: <20091215103759.P97203@beagle.kn.op.dlr.de> <200912150812.35521.jhb@freebsd.org> <20091215183859.S53283@beagle.kn.op.dlr.de> <200912151313.28326.jhb@freebsd.org> <20091219112711.GR55913@acme.spoerlein.net> <20091219154206.E93919@beagle.kn.op.dlr.de>
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On Sat, 19.12.2009 at 15:56:38 +0100, Harti Brandt wrote:
> On Sat, 19 Dec 2009, Ulrich Sprlein wrote:
>
> >On Tue, 15.12.2009 at 13:13:28 -0500, John Baldwin wrote:
> >> On Tuesday 15 December 2009 12:45:13 pm Harti Brandt wrote:
> >> > I see. I was also thinking along these lines, but was not sure whether it
> >> > is worth the trouble. I suppose this does not help to implement 64-bit
> >> > counters on 32-bit architectures, though, because you cannot read them
> >> > reliably without locking to sum them up, right?
> >>
> >> Either that or you just accept that you have a small race since it is only stats. :)
> >
> >This might be stupid, but can we not easily *read* 64bit counters
> >on 32bit machines like this:
> >
> >do {
> > h1 = read_upper_32bits;
> > l1 = read_lower_32bits;
> > h2 = read_upper_32bits;
> > l2 = read_lower_32bits; /* not needed */
> >} while (h1 != h2);
> >
> >sum64 = (h1<<32) + l1;
> >
> >or something like that? If h2 does not change between readings, no
> >wrap-around has occured. If l1 was read in between the readings of h1
> >and h2, the code above is sound. Right?
>
> I suppose this works only if it would be guaranteed that the CPU modifying
> the 64-bit value does this somehow faster than the CPU reading the data:
>
> CPU1 CPU2
> ---- ----
> write new h
> read h1 (new h)
> read l1 (old l)
> read h2 (new h)
> write new l
>
> It doesn't work too when the CPU first writes L and the H.
To be honest, I didn't even think about the 64 bit writes being
non-atomic, too. So, of course my suggestion was way too naive.
Also thanks to Bruce for re-iterating the whole write/read ordering
stuff yet again. :)
Regards,
Uli
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