site_archiver@lists.apple.com Delivered-To: darwin-dev@lists.apple.com Dkim-signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=beta; h=domainkey-signature:received:received:message-id:date:from:to:subject:in-reply-to:mime-version:content-type:content-transfer-encoding:content-disposition:references; bh=odpqrW7NQzcxtoGVP3WzYkQW7zfoGqlxuLlpVhG45E0=; b=Q61rMSRlHveI34ADf8IV6j1i0MERxXcSBjErKS4cIhU09wZPrtnT5d++ajTnbieSrVUMtyC92cIt4KxWEn+3LiOSBH3Xg2xwagoUOHOhpoJVZLloXSIwrZIMzVGpLLxh1ydxVoZsdj3mqYvS7aIjd22DpZHVY+fV2nwbknTTuNU= Domainkey-signature: a=rsa-sha1; c=nofws; d=gmail.com; s=beta; h=message-id:date:from:to:subject:in-reply-to:mime-version:content-type:content-transfer-encoding:content-disposition:references; b=Unlin8fVSBmRCvzL0BB58f/U1W8bWjxCgb/f/ADIZcj/b00Q6eZ2470WBdA+S7az4BOIYFTOr/oHNlSi+k70XtwFrzpQWhgrV/EOyPWlbwaHz6T+UC8yYSkUxxJ9oesrnpUMFF4qMaU4XimQmqd49awP5ZGeplq1HgdfuZVEq60= On Fri, Mar 28, 2008 at 10:12 PM, Michael Smith <msmith@freebsd.org> wrote:

On Mar 28, 2008, at 12:05 PM, darwin-dev-request@lists.apple.com wrote:

...

...

If I understood what you were actually trying to do, I might have some better ideas...

I'm not really sure what more I can say. I have a tight loop which sends packets at various user defined intervals. Speeds range from quite slow (a few packets/second) to quite fast (beyond 150K/sec). I'm hoping to send packets at equal distant intervals for an even distribution rather then time-slicing it (send a bunch of packets at once and sleeping for longer periods of time), but I suppose I may be asking too much.

Given that the packets are going to be clustered up by the time they hit the wire due to queueing and scheduling latencies in both the network stack and the MAC, I believe you are trying a bit too hard.

Pick a quantum that's not too coarse (a millisecond is probably a good place to start) and barf out a bunch of packets each time it ticks over.

Reasonable theory... Honestly, something I've given some thought to, but frankly, it's painfully hard to measure how much better/worse it would work. Putting a sniffer on the wire doesn't help since the receiving box is going to queue up the reads and I know pcap timestamps are inaccurate (It's not that uncommon to see sequential packet timestamps go backwards). At some point, you just have to do your best and be done with it.

You can tune the actual quantum number up or down based on a bunch of factors, but bear in mind that you're talking to a networking stack designed for user applications, not a SmartBits. 8) It would probably be educational to watch the actual timing of the resulting wire traffic (or if you're doing end-to-end work, timestamps on the received packets) to see how adjusting the quantum affects what you're actually trying to achieve.

If you are worried about beat effects, consider using a balanced PRNG to jitter your quantum around by a significant factor (I'd start at maybe +/- 30%). Then you can either leave the ppq value the same and get non-constant short term packet rates (but keeping the same long- term rate), or scale the ppq with the jitter factor to keep the short term packet rate constant but moving the bursts around to avoid e.g. scheduling quantum boundaries in whatever you're talking to.

One issue I'm dealing with is that I sometimes generate traffic *through* devices- meaning both client & server side of the connection. Beat effects are definitely an issue here and I'm not sure adding some jitter is a workable solution. Probably worth looking into sometime though. Using such a mechanisim could possibly allow higher packet rates.

If it's critical that your quantum be particularly small, you might also want to consider using code inside the kernel; this can save you on the per-packet overheads you're currently paying for pushing them into the kernel. In the most extreme case, you could steal an interface from the network stack completely, again depending on what your ultimate goals might be.

Cross platform is probably one of the biggest goals. I develop on OS X and most of my users use Linux, with some Solaris and *BSD thrown in for good measure. Maybe one of these days I'll do kernel modules, but for now I've got higher priorities. Right now my plan is to use weighted averaging to simulate sub 1usec accuracy. Ie, if the rate should be 7.7usec, I'll send 7 packets @ 8usec and 3 at 7usec.

HTH.

Definitely... always appreciate some ideas. -- Aaron Turner http://synfin.net/ http://tcpreplay.synfin.net/ - Pcap editing & replay tools for Unix They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety. -- Benjamin Franklin _______________________________________________ Do not post admin requests to the list. They will be ignored. Darwin-dev mailing list (Darwin-dev@lists.apple.com) Help/Unsubscribe/Update your Subscription: http://lists.apple.com/mailman/options/darwin-dev/site_archiver%40lists.appl... This email sent to site_archiver@lists.apple.com