Re: Stable tag slipped
Matthew Dillon wrote:
> :The machine doing the hammering is a Mac running MacOSX.
> :[pomme:~] raphael% sysctl net.inet.tcp.msl
> :net.inet.tcp.msl: 600
> :[pomme:~] raphael% sysctl kern.ipc.maxsockets
> :kern.ipc.maxsockets: 512
> :
> :Just keep in mind there is one loop running locally on the dfbsd machine
> :being tested.
> :
> :Why should the portrange be adjusted?
> :
> :Raphael
>
> The portrange is used to select the port number to use for an outgoing
> connection (it doesn't apply to incoming connections). Most programs
> use the standard first - last range, which is typically around 4000 ports.
>
> This is usually not a problem but if an originating machine is making a
> lot of rapid outgoing tcp connections to the same target (e.g. the MacOSX
> box making connections to a single web server), it can run out of ports
> because sockets often need a cool-down time (2*MSL = 60 seconds typ)
> before they can be reused. I don't know what the MSL default is on
> the Mac.
>
> Only one side of a TCP connection needs to go through the cooldown,
> but another issue is that the side chosen depends on which side
> disconnects first. And if both sides disconnect at the same time both
> can wind up in TIME_WAIT.
>
> So, for example, my own test program (which I am about to commit in
> /usr/src/test/stress/webstress) is causing over 5600 sockets to be
> in a TIME_WAIT state on the client and 300ish sockets in a TIME_WAIT
> state on the web server. That's with the MSL reduced to 15000 mS
> and net.inet.ip.portrange.last bumped up to 30000.
>
> Under normal circumstances an HTTP session uses the HTTP/1.1 protocol
> and the client does the disconnect, so TIME_WAIT sockets do not usually
> build up on the server. HTTP/1.0 (old style) connections often result
> in the server doing the disconnect first which often leaves it saddled
> with the TIME_WAIT socket. SMTP connections to a mail server almost
> universally leave the mail server saddled with the TIME_WAIT socket.
Thanks for the detailed explanation Matt.
So far, no hint of a problem on that machine, but I haven't tested the
scenario of long running tcp connections moving big files. (I have no
clue if this is relevant, I'm just guessing)
I am now looking for a way to do that without stressing too much the
disks, not to mention them being a bottleneck. I'm thinking of doing
that from a ramdisk.
by the way, will I be able to compile your webstress on MacOSX?
but for now I'm going to bed ;)
Raphael
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