Hi,
Just my 2-cents, but we've found polling to be extremely valuable on
low-end hardware as described here.
We use it only on fxp drivers, but it moved throughput on 133Mhz
hardware from something around 8Mbps to 20Mbps on regular layer-3
packet forwarding and also bumped VPN performance up significantly
when used with hardware VPN accelerators.
Also, since you can force the system to reserve a certain percent for
non-polling tasks, you still have SSH access even when it is running
hard, although I suppose at the risk of perhaps packet loss if your
userland applications use a lot of CPU.
I have no idea on FBSD 5.4 or 6.0 though and would be curious. We're
running 5.4 on our normal servers now, but I've been nervous moving
the low end equipment off of 4.x because of performance issues.
- mike
Michael F. DeMan
Director of Technology
OpenAccess Network Services
Bellingham, WA 98225
[EMAIL PROTECTED]
360-647-0785
On Feb 5, 2006, at 9:03 PM, Nate Nielsen wrote:
Sam Leffler wrote:
You might try explaining why you think polling helps your
performance.
Unless you've significantly restructured the interrupt handling in
the
driver most work is deferred to a non-interrupt context.
Yes, I saw that. However the interrupts themselves when they are fired
at upwards of a thousand per second do greatly affect performance and
scheduling on a slow box.
I understand that it may not seem intuitive on faster systems which
are
completely capable of handling such a small interrupt load. But these
embedded systems are slow little puppies, running between 133 and 266
Mhz (586 class chip).
Adding polling to this driver does increase performance on embedded
systems. With my current patch (on a 233Mhz system), the throughput
(in
this case a simple TCP stream) goes up by ~6Mbits, from 18Mbits to
24Mbits.
But that's not the main benefit. The main thing is the scheduling.
Without having the thousands of interrupts, the box is better able to
balance the RX/TX with the other aspects, such as encapsulation,
packet
filtering and other activities.
When the entire box is polling driven, it's total throughput
(ethernet,
encapsulation, hardware encryption, wireless) increases greatly and
does
not exhibit livelock symptoms.
Without polling, these slow systems easily exhibit livelock. This is
where incoming traffic can cause so many interrupts that outgoing
traffic is completely halted and the throughput drops to zero or near
zero. Under these conditions userland processes also run barely or not
at all. The scheduler has no say in what's going on in the system, as
interrupts overwhelm all other activity.
Also the
driver in 6.0 and later does tx interrupt mitigation and rx interrupt
coalescing so I wouldn't expect interrupt handling to be a
performance
limitation.
Interesting. If there's an option to enable it, I very well may have
missed it.
However it should be noted, that the default behaviour (in 6.0
release)
seems to be that the hardware generates about around 2000
interrupts per
second at around 15 - 18 Mbits throughput.
There are other issues that can affect performance but you
haven't mentioned them...
Obviously these are not mainstream performance issues for people just
trying to connect to an access point. But when the atheros cards are
used in backhaul applications and are running in the low power
embedded
systems you typically see on an antenna mast, polling makes a big
difference.
Cheers,
Nate
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