xl(4) & polling

Jon Simola jsimola at gmail.com
Tue May 10 23:33:03 PDT 2005

On 5/10/05, Rob <spamrefuse at yahoo.com> wrote:
> Interestingly: HZ=1000 is apparently a problem with
> the xl devices (3Com 3c905B-TX), but not with the
> rl devices (RealTek 8139).
> What could cause that difference? Could a difference
> in buffer size on the LAN card cause this?

Yes. GigE cards tend to have larger packet buffers, but that certainly
doesn't solve all the problems. I've been having some problems with
the em cards in particular (fxp I've had no problems with) as no
matter what I've tried tuning (tcprecvspace, HZ, polling knobs) I've
been seeing packet loss of about 0.5%. That doesn't seem like much,
but it's an awful lot to the couple thousand users behind it.

Anyways, HZ=1000 shouldn't be a CPU problem on anything faster than a
500MHz-ish processor. There are also a few lightly documented sysctls
that might be useful to play with that do things like poll during the
idle loop (actual usefullness in any particular case may be void in
your area, many will enter, few will win).

Jon Simola
Systems Administrator
ABC Communications

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