[Cerowrt-devel] Intel latency issue
stephen at networkplumber.org
Mon Dec 5 12:10:04 EST 2016
On Sun, 4 Dec 2016 14:41:05 -0500 (EST)
dpreed at reed.com wrote:
> The language used in the article seems confused. However, since firmware sometimes means software (the OS kernel, for example) and this is "lag under load", it's barely possible that this is bufferbloat of a sort, it seems. Would we be surprised?
> 200 ms. can also be due to interrupt mishandling, recovered by a watchdog. It's common for performance to reduce interrupt overhead by switching from interrupt driven to polled while packets are arriving at full rate and then back again when the traffic has a gap. If you don't turn interrupts back on correctly (there's a race between turning on interrupts and packet arrival after you decide and before you succeed in turning on interrupts), then you end up waiting for some "watchdog" (every 200 ms?) to handle the incoming packets.
> The idea that something actually runs for 200 ms. blocking everything seems to be the least likely situation - of course someone might have written code that held a lock while waiting for something or masked interrupts while waiting for something. But actually executing code for 200 ms.? Probably not.
I have some recent experience with this hw. At my new location, the ISP provided modem was a Hitron CGNv4
which earned an F on bufferbloat. New hardware is Arris Surboard SB6190 and Linksys AC3200 which now
gets all A's for bandwith, latency, and bufferbloat. The wireless is also much better.
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