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Re: Route cache performance under stress

To: "David S. Miller" <davem@xxxxxxxxxx>
Subject: Re: Route cache performance under stress
From: Ben Greear <greearb@xxxxxxxxxxxxxxx>
Date: Tue, 10 Jun 2003 18:51:17 -0700
Cc: netdev@xxxxxxxxxxx
In-reply-to: <20030610.182234.74725315.davem@xxxxxxxxxx>
Organization: Candela Technologies
References: <3EE67D2D.80608@xxxxxxxxxxxxxxx> <20030610.180120.71112140.davem@xxxxxxxxxx> <3EE682B8.8060708@xxxxxxxxxxxxxxx> <20030610.182234.74725315.davem@xxxxxxxxxx>
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David S. Miller wrote:

   Do we know when we are being asked for this value?

We have to take the timestamp at netif_receive_skb() for it to
be accurate.

We don't even know if this packet is for this host until a long
time later, let alone whether any local sockets want SO_RECVSTAMP
or whether any IP options want timestamp or whether tcpdump is
listening etc.

Yes, I understand why we want a time-stamp very early...but if
we can get _some_ sort of time stamp very cheap (TSC, for example)
then we can potentially defer the more expensive conversion of
this stamp into the equivalent of whatever do_gettimeofday will
give us.

We could set an 'is-timestamp-converted-already' flag on the skb and have
a macro that gets the timestamp.  This macro can do the conversion
as needed and return the value to calling code.  For platforms that do
not support TSC or it's equivalent, can just use gettimeofday for the
original stamp and set the flag..

Ben Greear <greearb@xxxxxxxxxxxxxxx>       <Ben_Greear AT>
President of Candela Technologies Inc

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