Dell & parport & Atheros WLAN latency woes

Hey y'all,

I have LinuxCNC 2.7.8 newly set up on a new to me Dell Opteron 780 (Core 2 Quad @ 2.5GHz, 4Gb memory, internal video), and I'm really struggling with latency. I'm using the parport for now, though I have a Mesa 5i25 & 7i76 coming.

Now, I've tried the isolcpu and IRQ affinity trick. This drops jitter on latency-test down to approximately 27us, and if I use idle=poll, I get down to ~7us. I'm loath to leave the machine running day in and day out with idle=poll, as it seems to gobble power - the cores all run at ~60C instead of ~35C. I've played with idle=halt (doesn't help), idle=mwait (doesn't boot) and idle=nomwait (probably a noop) without improvement over no idle argument.

So, I'd resigned myself to 27us of jitter, no biggie, I guess.

Enter networking problems.

I got a Ralink 7601 USB Wifi dongle, but the vendor's driver seems to be a piece of @#$%. After futzing with it for days, I find the ping times to my local router will creep up to 10s of seconds without any particular provocation - sad. The Linux community driver requires kernel 3.19 (LinuxCNC is at 3.4), and I really didn't want to dive into the rathole of trying to backport it.

So, I ran out to a local PC hardware store, and bought the oldest looking PCI WiFi card they had. This was a TP Link TL-WN851ND, which seems to be an Atheron chipset. To my relief, this just worked(TM) as soon as I'd plugged it in. The only problem is that it bumps my latency to >300us. This time-warp seems to happen with a period >100s, but I can provoke it by doing "ifconfig up wlan0".

Here's the lscpi for it:
--- cut here ---
# lspci -s 04:02.0 -vv -k
04:02.0 Network controller: Atheros Communications Inc. AR9227 Wireless Network Adapter (rev 01)
Subsystem: Atheros Communications Inc. Device 0300
Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV+ VGASnoop- ParErr- Stepping- SERR+ FastB2B- DisINTx-
Status: Cap+ 66MHz+ UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
Latency: 168, Cache Line Size: 64 bytes
Interrupt: pin A routed to IRQ 18
Region 0: Memory at f76f0000 (32-bit, non-prefetchable) [size=64K]
Capabilities: [40] #80 [0000]
Kernel driver in use: ath9k
--- cut here ---

It seems the time warp goes away when I "ifconfig down wlan0", so that'll be the short term fix, though it's a PITA as I post GCode from another machine through Samba ATM.

So, to my questions; 27us - is that OK for parport? Hopefully it be fine for the Mesa 5i25 card? Am I going to be better off with another Linux version/distro with the Mesa card?

For the Atheros, is this a known issue? Is there any fix that don't involve turning down networking?

Siggi

siggi wrote:

The only problem is that it bumps my latency to >300us. This time-warp seems to happen with a period >100s, but I can provoke it by doing "ifconfig up wlan0".

I forgot to mention that I thought installing the card had provoked SMI somehow, so I tried the "smictrl" utility, which has no effect.

tommylight wrote:

Optiplex 780, i think.

Doh, of course. Thanks...