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Re: Route cache performance under stress

To: "David S. Miller" <davem@xxxxxxxxxx>, "'netdev@xxxxxxxxxxx'" <netdev@xxxxxxxxxxx>
Subject: Re: Route cache performance under stress
From: Ben Greear <greearb@xxxxxxxxxxxxxxx>
Date: Tue, 10 Jun 2003 18:15:36 -0700
In-reply-to: <20030610.180120.71112140.davem@redhat.com>
Organization: Candela Technologies
References: <20030610.152020.59678979.davem@redhat.com> <Pine.LNX.4.51.0306101956520.7801@ns.istop.com> <3EE67D2D.80608@candelatech.com> <20030610.180120.71112140.davem@redhat.com>
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David S. Miller wrote:
From: Ben Greear <greearb@xxxxxxxxxxxxxxx>
Date: Tue, 10 Jun 2003 17:51:57 -0700
Maybe as a configurable option, since it would make tcpdump less useful.
Seems like we could kludge it up so that we used the TSC (or whatever that
really fast hardware clock is) to provide some relative stamp that could be
converted to a time_val later?


I have a strange feeling that Ralph's system isn't using
TSC and that's why it shows up so high on the profiles :-)
TSC do_gettimeofday() is REALLY cheap (TSC read plus a multiply which
x86 does in like 5 cycles).

Yes, this idea has been tossed around before.  But what's funny
is that on the bigger boxes, you don't use TSC because amongst
the different nodes of the machine they are skewed, so you have
to use the ACPI timer or something like that for timestamping.

What determines whether or not we use the "TSC do_gettimeofday". Does it automagically happen when you compile for P-III or something like that?

And how big of a "bigger box" are you talking about...regular old SMP, or NUMA?

It does seem a bit wasteful to do the gettimeofday when most of the time
the result is ignored.
(Or, are there things other than tcpdump that need the gettimeofday stamp?)


SO_RECVSTAMP, any socket on the machine can ask for this.

Do we know when we are being asked for this value? Ie, could we do the TSC/ACPI timer -> time_val conversion here? If TSC means cheap gettimeofday anyway, then my last question is moot.

Thanks,
Ben


-- Ben Greear <greearb@xxxxxxxxxxxxxxx> <Ben_Greear AT excite.com> President of Candela Technologies Inc http://www.candelatech.com ScryMUD: http://scry.wanfear.com http://scry.wanfear.com/~greear



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