Date: Thu, 12 Apr 2012 09:54:30 +0200 From: Luigi Rizzo <rizzo@iet.unipi.it> To: Bruce Evans <brde@optusnet.com.au> Cc: Barney Wolff <barney@databus.com>, Andre Oppermann <andre@freebsd.org>, current@freebsd.org, net@freebsd.org Subject: Re: strange ping response times... Message-ID: <20120412075430.GA71623@onelab2.iet.unipi.it> In-Reply-To: <20120412112401.Q981@besplex.bde.org> References: <20120410225257.GB53350@onelab2.iet.unipi.it> <4F84B6DB.5040904@freebsd.org> <20120410230500.GA22829@pit.databus.com> <20120410233211.GA53829@onelab2.iet.unipi.it> <4F855E5E.5000107@freebsd.org> <20120411110036.GA60031@onelab2.iet.unipi.it> <4F857631.8040309@freebsd.org> <20120411131920.GA61027@onelab2.iet.unipi.it> <20120412112401.Q981@besplex.bde.org>
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On Thu, Apr 12, 2012 at 01:18:59PM +1000, Bruce Evans wrote:
> On Wed, 11 Apr 2012, Luigi Rizzo wrote:
>
> >On Wed, Apr 11, 2012 at 02:16:49PM +0200, Andre Oppermann wrote:
...
> >ping takes a timestamp in userspace before trying to transmit
> >the packet, and then the timestamp for the received packet
> >is recorded in the kernel (in the interrupt or netisr thread
> >i believe -- anyways, not in userspace).
>
> No, all timestamps recorded used by ping are recorded in userland.
Bruce, look at the code in ping.c -- SO_TIMESTAMP is defined,
so the program does (successfully) a
setsockopt(s, SOL_SOCKET, SO_TIMESTAMP, &on, sizeof(on));
and then (verified that at runtime the code follows this path)
struct cmsghdr *cmsg = (struct cmsghdr *)&ctrl;
msg.msg_controllen = sizeof(ctrl);
msg.msg_namelen = sizeof(from);
if ((cc = recvmsg(s, &msg, 0)) < 0) {
if (errno == EINTR)
continue;
warn("recvmsg");
continue;
}
if (cmsg->cmsg_level == SOL_SOCKET &&
cmsg->cmsg_type == SCM_TIMESTAMP &&
cmsg->cmsg_len == CMSG_LEN(sizeof *tv)) {
/* Copy to avoid alignment problems: */
memcpy(&now, CMSG_DATA(cmsg), sizeof(now));
tv = &now;
}
...
> My normal ping -fq localhost RTT is 2-3 usec
> (closer to 3; another bug in this area is that the timestamps only
> have microseconds resolution so you can't see if 3 is actually
> more like 2.5. I was thinking of changing the resolution to
> nanoseconds 8-10 years ago, before the FreeBSD-5 pessimizations
> and CPU speeds hitting a wall made this not really necessary),
> but the kernel I'm testing with uses ipfw which bloats the RTT to 8-9
ipfw overhead obviously depends on your firewall configuration,
i have tested without ipfw to remove that noise.
cheers
luigi
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