On 2024-08-12 16:19, Stanislav Fomichev wrote:
On 08/12, Joe Damato wrote:
Greetings:
Martin Karsten (CC'd) and I have been collaborating on some ideas about
ways of reducing tail latency when using epoll-based busy poll and we'd
love to get feedback from the list on the code in this series. This is
the idea I mentioned at netdev conf, for those who were there. Barring
any major issues, we hope to submit this officially shortly after RFC.
The basic idea for suspending IRQs in this manner was described in an
earlier paper presented at Sigmetrics 2024 [1].
Let me explicitly call out the paper. Very nice analysis!
Thank you!
[snip]
Here's how it is intended to work:
- An administrator sets the existing sysfs parameters for
defer_hard_irqs and gro_flush_timeout to enable IRQ deferral.
- An administrator sets the new sysfs parameter irq_suspend_timeout
to a larger value than gro-timeout to enable IRQ suspension.
Can you expand more on what's the problem with the existing gro_flush_timeout?
Is it defer_hard_irqs_count? Or you want a separate timeout only for the
perfer_busy_poll case(why?)? Because looking at the first two patches,
you essentially replace all usages of gro_flush_timeout with a new variable
and I don't see how it helps.
gro-flush-timeout (in combination with defer-hard-irqs) is the default
irq deferral mechanism and as such, always active when configured. Its
static periodic softirq processing leads to a situation where:
- A long gro-flush-timeout causes high latencies when load is
sufficiently below capacity, or
- a short gro-flush-timeout causes overhead when softirq execution
asynchronously competes with application processing at high load.
The shortcomings of this are documented (to some extent) by our
experiments. See defer20 working well at low load, but having problems
at high load, while defer200 having higher latency at low load.
irq-suspend-timeout is only active when an application uses
prefer-busy-polling and in that case, produces a nice alternating
pattern of application processing and networking processing (similar to
what we describe in the paper). This then works well with both low and
high load.
Maybe expand more on what code paths are we trying to improve? Existing
busy polling code is not super readable, so would be nice to simplify
it a bit in the process (if possible) instead of adding one more tunable.
There are essentially three possible loops for network processing:
1) hardirq -> softirq -> napi poll; this is the baseline functionality
2) timer -> softirq -> napi poll; this is deferred irq processing scheme
with the shortcomings described above
3) epoll -> busy-poll -> napi poll
If a system is configured for 1), not much can be done, as it is
difficult to interject anything into this loop without adding state and
side effects. This is what we tried for the paper, but it ended up being
a hack.
If however the system is configured for irq deferral, Loops 2) and 3)
"wrestle" with each other for control. Injecting the larger
irq-suspend-timeout for 'timer' in Loop 2) essentially tilts this in
favour of Loop 3) and creates the nice pattern describe above.
[snip]
- suspendX:
- set defer_hard_irqs to 100
- set gro_flush_timeout to X,000
- set irq_suspend_timeout to 20,000,000
- enable busy poll via the existing ioctl (busy_poll_usecs = 0,
busy_poll_budget = 64, prefer_busy_poll = true)
What's the intention of `busy_poll_usecs = 0` here? Presumably we fallback
to busy_poll sysctl value?
Before this patch set, ep_poll only calls napi_busy_poll, if busy_poll
(sysctl) or busy_poll_usecs is nonzero. However, this might lead to
busy-polling even when the application does not actually need or want
it. Only one iteration through the busy loop is needed to make the new
scheme work. Additional napi busy polling over and above is optional.
Thanks,
Martin
