Messages in this thread | | | Date | Thu, 20 Feb 2014 13:16:18 -0500 | From | Peter Hurley <> | Subject | Re: locking changes in tty broke low latency feature |
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On 02/19/2014 09:55 PM, Peter Hurley wrote: > On 02/19/2014 06:06 PM, Hal Murray wrote: >>> Can you give me an idea of your device's average and minimum required >>> latency (please be specific)? Is your target arch x86 [so I can evaluate the >>> the impact of bus-locked instructions relative to your expected]? >> >> The code I'm familiar with is ntpd and gpsd. They run on almost any hardware >> or OS and talk to a wide collection of devices. >> >> There is no hard requirement for latency. They just work better with lower >> latency. The lower the better. >> >> People gripe about the latency due to USB polling which is about a ms. > > Have you tried 3.12+ without low_latency? I ripped out a lot of locks > from 3.12+ so it's possible it already meets your requirements.
Using Alan's idea to mock up a latency test, I threw together a test jig using two computers running 3.14-rc1 and my fwserial driver (modified to not aggregrate writes) in raw mode where the target does this:
while (1) { read 64 bytes compare to pattern write 1 byte response }
and the sender does this:
for (i = 0; i < 2000; i++) { write 64-byte pattern read 1 byte response }
Sender completes 2000 loops in 160ms total run time; that's 80us average per complete round-trip.
Here's a snapshot of a function trace for 1 complete round trip at the target:
<idle>-0 [007] d.h1 4856.935561: irq_handler: firewire_ohci 0000:07:06.0: IRQ 00000010 AR_req <idle>-0 [007] ..s1 4856.935566: log_ar_at_event.isra.33: firewire_ohci 0000:07:06.0: AR spd 2 tl 2a, ffc1 -> ffc0, ack_complete, BW req, 800000000004 40,0 <idle>-0 [007] ..s1 4856.935567: tty_flip_buffer_push <-fwtty_port_handler kworker/7:0-6872 [007] .... 4856.935571: flush_to_ldisc <-process_one_work tty-latency-tes-6891 [003] .... 4856.935578: n_tty_write <-tty_write <idle>-0 [003] d.h1 4856.935591: irq_handler: firewire_ohci 0000:07:06.0: IRQ 00000001 AT_req <idle>-0 [003] ..s1 4856.935594: log_ar_at_event.isra.33: firewire_ohci 0000:07:06.0: AT spd 2 tl 12, ffc0 -> ffc1, ack_complete, BW req, 800000000004 1,0
The same snapshot explained:
The firewire controller IRQ for received packet in rx dma fifo:
<idle>-0 [007] d.h1 4856.935561: irq_handler: firewire_ohci 0000:07:06.0: IRQ 00000010 AR_req
The firewire-ohci driver rx fifo tasklet running:
<idle>-0 [007] ..s1 4856.935566: log_ar_at_event.isra.33: firewire_ohci 0000:07:06.0: AR spd 2 tl 2a, ffc1 -> ffc0, ack_complete, BW req, 800000000004 40,0
The fwserial driver schdeduling the rx data:
<idle>-0 [007] ..s1 4856.935567: tty_flip_buffer_push <-fwtty_port_handler
The tty buffer input worker running:
kworker/7:0-6872 [007] .... 4856.935571: flush_to_ldisc <-process_one_work
The test application having received the data, compared it, and writing the 1-byte response:
tty-latency-tes-6891 [003] .... 4856.935578: n_tty_write <-tty_write
The firewire controller IRQ for running the tx dma fifo:
<idle>-0 [003] d.h1 4856.935591: irq_handler: firewire_ohci 0000:07:06.0: IRQ 00000001 AT_req
The firewire-ohci driver tx fifo tasklet acknowledging the sender of the 64-byte packet has received the 1-byte response:
<idle>-0 [003] ..s1 4856.935594: log_ar_at_event.isra.33: firewire_ohci 0000:07:06.0: AT spd 2 tl 12, ffc0 -> ffc1, ack_complete, BW req, 800000000004 1,0
... trace continued for another 4x 64-byte reads and 1 byte responses:
<idle>-0 [007] d.h1 4856.935646: irq_handler: firewire_ohci 0000:07:06.0: IRQ 00000010 AR_req <idle>-0 [007] ..s1 4856.935650: log_ar_at_event.isra.33: firewire_ohci 0000:07:06.0: AR spd 2 tl 2b, ffc1 -> ffc0, ack_complete, BW req, 800000000004 40,0 <idle>-0 [007] ..s1 4856.935651: tty_flip_buffer_push <-fwtty_port_handler kworker/7:0-6872 [007] .... 4856.935655: flush_to_ldisc <-process_one_work tty-latency-tes-6891 [003] .... 4856.935662: n_tty_write <-tty_write <idle>-0 [003] d.h1 4856.935674: irq_handler: firewire_ohci 0000:07:06.0: IRQ 00000001 AT_req <idle>-0 [003] ..s1 4856.935678: log_ar_at_event.isra.33: firewire_ohci 0000:07:06.0: AT spd 2 tl 13, ffc0 -> ffc1, ack_complete, BW req, 800000000004 1,0 <idle>-0 [007] d.h1 4856.935729: irq_handler: firewire_ohci 0000:07:06.0: IRQ 00000010 AR_req <idle>-0 [007] ..s1 4856.935733: log_ar_at_event.isra.33: firewire_ohci 0000:07:06.0: AR spd 2 tl 2c, ffc1 -> ffc0, ack_complete, BW req, 800000000004 40,0 <idle>-0 [007] ..s1 4856.935734: tty_flip_buffer_push <-fwtty_port_handler kworker/7:0-6872 [007] .... 4856.935738: flush_to_ldisc <-process_one_work tty-latency-tes-6891 [003] .... 4856.935746: n_tty_write <-tty_write <idle>-0 [003] d.h1 4856.935758: irq_handler: firewire_ohci 0000:07:06.0: IRQ 00000001 AT_req <idle>-0 [003] ..s1 4856.935762: log_ar_at_event.isra.33: firewire_ohci 0000:07:06.0: AT spd 2 tl 14, ffc0 -> ffc1, ack_complete, BW req, 800000000004 1,0 <idle>-0 [007] d.h1 4856.935812: irq_handler: firewire_ohci 0000:07:06.0: IRQ 00000010 AR_req <idle>-0 [007] ..s1 4856.935816: log_ar_at_event.isra.33: firewire_ohci 0000:07:06.0: AR spd 2 tl 2d, ffc1 -> ffc0, ack_complete, BW req, 800000000004 40,0 <idle>-0 [007] ..s1 4856.935817: tty_flip_buffer_push <-fwtty_port_handler kworker/7:0-6872 [007] .... 4856.935821: flush_to_ldisc <-process_one_work tty-latency-tes-6891 [003] .... 4856.935828: n_tty_write <-tty_write <idle>-0 [003] d.h1 4856.935844: irq_handler: firewire_ohci 0000:07:06.0: IRQ 00000001 AT_req <idle>-0 [003] ..s1 4856.935847: log_ar_at_event.isra.33: firewire_ohci 0000:07:06.0: AT spd 2 tl 15, ffc0 -> ffc1, ack_complete, BW req, 800000000004 1,0 <idle>-0 [007] d.h1 4856.935895: irq_handler: firewire_ohci 0000:07:06.0: IRQ 00000010 AR_req <idle>-0 [007] ..s1 4856.935899: log_ar_at_event.isra.33: firewire_ohci 0000:07:06.0: AR spd 2 tl 2e, ffc1 -> ffc0, ack_complete, BW req, 800000000004 40,0 <idle>-0 [007] ..s1 4856.935900: tty_flip_buffer_push <-fwtty_port_handler kworker/7:0-6872 [007] .... 4856.935904: flush_to_ldisc <-process_one_work tty-latency-tes-6891 [003] .... 4856.935911: n_tty_write <-tty_write <idle>-0 [003] d.h1 4856.935923: irq_handler: firewire_ohci 0000:07:06.0: IRQ 00000001 AT_req <idle>-0 [003] ..s1 4856.935927: log_ar_at_event.isra.33: firewire_ohci 0000:07:06.0: AT spd 2 tl 16, ffc0 -> ffc1, ack_complete, BW req, 800000000004 1,0
I think this shows that low_latency is unnecessary and should just be removed/ignored by the tty core.
Let me know if you want my test jig (or fwserial patch). The test was run on a 10-year old Vaio single-core laptop as the sender and 6-year old dual socket xeon workstation (so nothing super tricked out for this test).
Regards, Peter Hurley
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