Stop using tasklets for bottom halves

Stop using tasklets for bottom halves

[Luis R. Rodriguez]

A while ago I had read about an effort to consider removing tasklets
[1] or at least trying to not use them. I'm unaware of the progress in
this respect but since reading that article have always tried to
evaluate whether or not we need tasklets on wireless drivers. I have
also wondered whether work in irq context in other parts of the kernel
can be moved to process context, a curious example being timers. I'll
personally be trying to using only process context on bottom halves on
future drivers but I figured it may be a good time to ask how serious
was avoiding tasklets or using wrappers in the future to avoid irq
context is or is it advised. Do we have a general agreement this is a
good step forward to take? Has anyone made tests or changes on a
specific driver from irq context to process context and proven there
are no significant advantages of using irq context where you would
have expected it?

Wireless in particular should IMHO not require taskets for anything
time sensitive that I can think about except perhaps changing channels
quickly and to do that appropriately also process pending RX frames
prior to a switch. It remains to be seen experimentally whether or not
using a workqueue for RX processing would affect the time to switch
channels negatively but I doubt it would be significant. I hope to
test that with ath9k_htc.

What about gigabit or 10 Gigabit Ethernet drivers ? Do they face any
challenges which would yet need to be proven would not face issues
when processing bottom halves in process context?

[1] http://lwn.net/Articles/239633/

  Luis
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Re: Stop using tasklets for bottom halves

[Stephen Hemminger]

On Mon, 7 Sep 2009 15:58:50 -0700
"Luis R. Rodriguez" <mcgrof@gmail.com> wrote:

> A while ago I had read about an effort to consider removing tasklets
> [1] or at least trying to not use them. I'm unaware of the progress in
> this respect but since reading that article have always tried to
> evaluate whether or not we need tasklets on wireless drivers. I have
> also wondered whether work in irq context in other parts of the kernel
> can be moved to process context, a curious example being timers. I'll
> personally be trying to using only process context on bottom halves on
> future drivers but I figured it may be a good time to ask how serious
> was avoiding tasklets or using wrappers in the future to avoid irq
> context is or is it advised. Do we have a general agreement this is a
> good step forward to take? Has anyone made tests or changes on a
> specific driver from irq context to process context and proven there
> are no significant advantages of using irq context where you would
> have expected it?
> 
> Wireless in particular should IMHO not require taskets for anything
> time sensitive that I can think about except perhaps changing channels
> quickly and to do that appropriately also process pending RX frames
> prior to a switch. It remains to be seen experimentally whether or not
> using a workqueue for RX processing would affect the time to switch
> channels negatively but I doubt it would be significant. I hope to
> test that with ath9k_htc.
> 
> What about gigabit or 10 Gigabit Ethernet drivers ? Do they face any
> challenges which would yet need to be proven would not face issues
> when processing bottom halves in process context?
> 
> [1] http://lwn.net/Articles/239633/
> 
>   Luis
> --
> To unsubscribe from this list: send the line "unsubscribe netdev" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

Why not use NAPI, which is soft irq? Almost all 1G and 10G drivers
use NAPI.

Process context is too slow.

-- 

Re: Stop using tasklets for bottom halves

[Steven Rostedt]

On Mon, 2009-09-07 at 17:14 -0700, Stephen Hemminger wrote:
> On Mon, 7 Sep 2009 15:58:50 -0700
> "Luis R. Rodriguez" <mcgrof-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> 
> > A while ago I had read about an effort to consider removing tasklets
> > [1] or at least trying to not use them. I'm unaware of the progress in
> > this respect but since reading that article have always tried to
> > evaluate whether or not we need tasklets on wireless drivers. I have
> > also wondered whether work in irq context in other parts of the kernel
> > can be moved to process context, a curious example being timers. I'll
> > personally be trying to using only process context on bottom halves on
> > future drivers but I figured it may be a good time to ask how serious
> > was avoiding tasklets or using wrappers in the future to avoid irq
> > context is or is it advised. Do we have a general agreement this is a
> > good step forward to take? Has anyone made tests or changes on a
> > specific driver from irq context to process context and proven there
> > are no significant advantages of using irq context where you would
> > have expected it?
> > 
> > Wireless in particular should IMHO not require taskets for anything
> > time sensitive that I can think about except perhaps changing channels
> > quickly and to do that appropriately also process pending RX frames
> > prior to a switch. It remains to be seen experimentally whether or not
> > using a workqueue for RX processing would affect the time to switch
> > channels negatively but I doubt it would be significant. I hope to
> > test that with ath9k_htc.
> > 
> > What about gigabit or 10 Gigabit Ethernet drivers ? Do they face any
> > challenges which would yet need to be proven would not face issues
> > when processing bottom halves in process context?
> > 
> > [1] http://lwn.net/Articles/239633/
> > 
> >   Luis
> > --
> > To unsubscribe from this list: send the line "unsubscribe netdev" in
> > the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> 
> Why not use NAPI, which is soft irq? Almost all 1G and 10G drivers
> use NAPI.
> 
> Process context is too slow.

Well, I'm hoping to prove the opposite. I'm working on some stuff that I
plan to present at Linux Plumbers. I've been too distracted by other
things, but hopefully I'll have some good numbers to present by then.

-- Steve


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Re: Stop using tasklets for bottom halves

[Luis R. Rodriguez]

On Mon, Sep 7, 2009 at 7:17 PM, Steven Rostedt<rostedt-nx8X9YLhiw1AfugRpC6u6w@public.gmane.org> wrote:
> On Mon, 2009-09-07 at 17:14 -0700, Stephen Hemminger wrote:
>> On Mon, 7 Sep 2009 15:58:50 -0700
>> "Luis R. Rodriguez" <mcgrof-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>>
>> > A while ago I had read about an effort to consider removing tasklets
>> > [1] or at least trying to not use them. I'm unaware of the progress in
>> > this respect but since reading that article have always tried to
>> > evaluate whether or not we need tasklets on wireless drivers. I have
>> > also wondered whether work in irq context in other parts of the kernel
>> > can be moved to process context, a curious example being timers. I'll
>> > personally be trying to using only process context on bottom halves on
>> > future drivers but I figured it may be a good time to ask how serious
>> > was avoiding tasklets or using wrappers in the future to avoid irq
>> > context is or is it advised. Do we have a general agreement this is a
>> > good step forward to take? Has anyone made tests or changes on a
>> > specific driver from irq context to process context and proven there
>> > are no significant advantages of using irq context where you would
>> > have expected it?
>> >
>> > Wireless in particular should IMHO not require taskets for anything
>> > time sensitive that I can think about except perhaps changing channels
>> > quickly and to do that appropriately also process pending RX frames
>> > prior to a switch. It remains to be seen experimentally whether or not
>> > using a workqueue for RX processing would affect the time to switch
>> > channels negatively but I doubt it would be significant. I hope to
>> > test that with ath9k_htc.
>> >
>> > What about gigabit or 10 Gigabit Ethernet drivers ? Do they face any
>> > challenges which would yet need to be proven would not face issues
>> > when processing bottom halves in process context?
>> >
>> > [1] http://lwn.net/Articles/239633/
>> >
>> >   Luis
>> > --
>> > To unsubscribe from this list: send the line "unsubscribe netdev" in
>> > the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
>> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
>>
>> Why not use NAPI, which is soft irq? Almost all 1G and 10G drivers
>> use NAPI.
>>
>> Process context is too slow.
>
> Well, I'm hoping to prove the opposite. I'm working on some stuff that I
> plan to present at Linux Plumbers. I've been too distracted by other
> things, but hopefully I'll have some good numbers to present by then.

What day in specific was this planned for at Plumbers?

  Luis
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Re: Stop using tasklets for bottom halves

[Steven Rostedt]

On Mon, 2009-09-07 at 21:16 -0700, Luis R. Rodriguez wrote:
> On Mon, Sep 7, 2009 at 7:17 PM, Steven Rostedt<rostedt-nx8X9YLhiw1AfugRpC6u6w@public.gmane.org> wrote:

> >>
> >> Process context is too slow.
> >
> > Well, I'm hoping to prove the opposite. I'm working on some stuff that I
> > plan to present at Linux Plumbers. I've been too distracted by other
> > things, but hopefully I'll have some good numbers to present by then.
> 
> What day in specific was this planned for at Plumbers?

Wednesday, during the networking session.

  http://linuxplumbersconf.org/ocw/proposals/53

-- Steve


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Re: Stop using tasklets for bottom halves

[Michael Buesch]

On Tuesday 08 September 2009 04:17:34 Steven Rostedt wrote:
> > Process context is too slow.
> 
> Well, I'm hoping to prove the opposite. I'm working on some stuff that I
> plan to present at Linux Plumbers. I've been too distracted by other
> things, but hopefully I'll have some good numbers to present by then.

I recently converted the b43 driver to threaded interrupt handlers and
a workqueue based TX mechanism. (My motivation was porting b43 to the SDIO bus that
needs to sleep, so requires process context).

There are two things that I noticed. When looking at the "idle" percentage in "top"
it regressed quite a bit when using threaded IRQ handlers. It shows about 8% less
idle. This is with threaded IRQs patched in, but without WQ TX mechanism. Applying
the WQ TX mechanism does not show any noticeable effect in "top".

I'm not quite sure where the 8% slowdown on threaded IRQ handlers come from. I'm not
really certain that it's _really_ a regression and not just a statistics accounting quirk.
Why does threaded IRQs slow down stuff and threaded TX does not at all? That does not
make sense at all to me.

I think there's no real reason for process context being slow in general. It's just that
we have additional context switches. But these are fast on Linux.

-- 
Greetings, Michael.
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Re: Stop using tasklets for bottom halves

[Michael Buesch]

On Tuesday 08 September 2009 06:50:41 Michael Buesch wrote:
> On Tuesday 08 September 2009 04:17:34 Steven Rostedt wrote:
> > > Process context is too slow.
> > 
> > Well, I'm hoping to prove the opposite. I'm working on some stuff that I
> > plan to present at Linux Plumbers. I've been too distracted by other
> > things, but hopefully I'll have some good numbers to present by then.
> 
> I recently converted the b43 driver to threaded interrupt handlers and
> a workqueue based TX mechanism. (My motivation was porting b43 to the SDIO bus that
> needs to sleep, so requires process context).
> 
> There are two things that I noticed. When looking at the "idle" percentage in "top"
> it regressed quite a bit when using threaded IRQ handlers. It shows about 8% less
> idle. This is with threaded IRQs patched in, but without WQ TX mechanism. Applying
> the WQ TX mechanism does not show any noticeable effect in "top".
> 
> I'm not quite sure where the 8% slowdown on threaded IRQ handlers come from. I'm not
> really certain that it's _really_ a regression and not just a statistics accounting quirk.
> Why does threaded IRQs slow down stuff and threaded TX does not at all? That does not
> make sense at all to me.
> 
> I think there's no real reason for process context being slow in general. It's just that
> we have additional context switches. But these are fast on Linux.
> 

Ok, I just did another test. I used a workqueue instead of the standard kernel threaded
IRQ infrastructure. Now the slowdown is only about 4% in "top". Maybe that shows room
for improvement in the threaded IRQ implementation...

B43 does call mac80211's "irqsafe" TX-status and RX functions. They schedule
additional tasklets. That is not required, however. Maybe I should remove that stuff and
retry my tests. That should also improve stuff a bit.

And yes, I notice that "top" is actually crap for testing performance issues. :)

-- 
Greetings, Michael.

Re: Stop using tasklets for bottom halves

[Ingo Molnar]

* Michael Buesch <mb@bu3sch.de> wrote:

> There are two things that I noticed. When looking at the "idle" 
> percentage in "top" it regressed quite a bit when using threaded 
> IRQ handlers. It shows about 8% less idle. This is with threaded 
> IRQs patched in, but without WQ TX mechanism. Applying the WQ TX 
> mechanism does not show any noticeable effect in "top".
> 
> I'm not quite sure where the 8% slowdown on threaded IRQ handlers 
> come from. I'm not really certain that it's _really_ a regression 
> and not just a statistics accounting quirk. Why does threaded IRQs 
> slow down stuff and threaded TX does not at all? That does not 
> make sense at all to me.

Do you have an x86 box to test it on?

If yes then perfcounters can be used for _much_ more precise 
measurements that you can trust. Do something like this:

   perf stat -a --repeat 3 sleep 1

The '-a/--all' option will measure all CPUs - everything: IRQ 
context, irqs-off region, etc. That output will be comparable before 
your threaded patch and after the patch.

Here's an example. I started one infinite loop on a testbox, which 
is using 100% of a single CPU. The system-wide stats look like this:

 # perf stat -a --repeat 3 sleep 1

 Performance counter stats for 'sleep 1' (3 runs):

   16003.320239  task-clock-msecs         #     15.993 CPUs    ( +-   0.044% )
             94  context-switches         #      0.000 M/sec   ( +-  11.373% )
              3  CPU-migrations           #      0.000 M/sec   ( +-  25.000% )
            170  page-faults              #      0.000 M/sec   ( +-  0.518% )
     3294001334  cycles                   #    205.832 M/sec   ( +-  0.896% )
     1088670782  instructions             #      0.331 IPC     ( +-  0.905% )
        1720926  cache-references         #      0.108 M/sec   ( +-  1.880% )
          61253  cache-misses             #      0.004 M/sec   ( +-  4.401% )

    1.000623219  seconds time elapsed   ( +-   0.002% )

the instructions count or the cycle count will go up or down, 
precisely according to how the threaded handlers. These stats are 
not time sampled but 'real', so they reflect reality and show 
whether your workload had to spend more (or less) cycles / 
instructions /etc.

I started a second loop in addition to the first one, and perf stat 
now gives me this output:

 # perf stat -a --repeat 3 sleep 1

 Performance counter stats for 'sleep 1' (3 runs):

   16003.289509  task-clock-msecs         #     15.994 CPUs    ( +-   0.046% )
             88  context-switches         #      0.000 M/sec   ( +-  15.933% )
              2  CPU-migrations           #      0.000 M/sec   ( +-  14.286% )
            188  page-faults              #      0.000 M/sec   ( +-   9.414% )
     6481963224  cycles                   #    405.039 M/sec   ( +-   0.011% )
     2152924468  instructions             #      0.332 IPC     ( +-   0.054% )
         397564  cache-references         #      0.025 M/sec   ( +-   1.217% )
          59835  cache-misses             #      0.004 M/sec   ( +-   3.732% )

    1.000576354  seconds time elapsed   ( +-   0.005% )

Compare the two results:

 before:
     6481963224  cycles                   #    405.039 M/sec   ( +-   0.011% )
     2152924468  instructions             #      0.332 IPC     ( +-   0.054% )

 after:
     3294001334  cycles                   #    205.832 M/sec   ( +-  0.896% )
     1088670782  instructions             #      0.331 IPC     ( +-  0.905% )

The cycles/sec doubled - as expected. You could do the same with 
your test and not have to rely in the very imprecise (and often 
misleading) 'top' statistics for kernel development.

The IPC (instructions per cycle) factor stayed roughly constant - 
showing that both workloads can push the same amount of instructions 
when normalized to a single CPU. If a workload becomes very 
cache-missy or executes a lot of system calls then the IPC factor 
goes down - if it becomes more optimal 'tight' code then the IPC 
factor goes up.)

(The cache-miss rate was very low in both cases - it's a simple 
infinite loop i tested.)

Furthermore the error bars in the rightmost column help you know 
whether any difference in results is statistically significant, or 
within the noise level.

Hope this helps,

	Ingo

Re: Stop using tasklets for bottom halves

[Stephen Hemminger]

On Mon, 07 Sep 2009 22:17:34 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Mon, 2009-09-07 at 17:14 -0700, Stephen Hemminger wrote:
> > On Mon, 7 Sep 2009 15:58:50 -0700
> > "Luis R. Rodriguez" <mcgrof@gmail.com> wrote:
> > 
> > > A while ago I had read about an effort to consider removing tasklets
> > > [1] or at least trying to not use them. I'm unaware of the progress in
> > > this respect but since reading that article have always tried to
> > > evaluate whether or not we need tasklets on wireless drivers. I have
> > > also wondered whether work in irq context in other parts of the kernel
> > > can be moved to process context, a curious example being timers. I'll
> > > personally be trying to using only process context on bottom halves on
> > > future drivers but I figured it may be a good time to ask how serious
> > > was avoiding tasklets or using wrappers in the future to avoid irq
> > > context is or is it advised. Do we have a general agreement this is a
> > > good step forward to take? Has anyone made tests or changes on a
> > > specific driver from irq context to process context and proven there
> > > are no significant advantages of using irq context where you would
> > > have expected it?
> > > 
> > > Wireless in particular should IMHO not require taskets for anything
> > > time sensitive that I can think about except perhaps changing channels
> > > quickly and to do that appropriately also process pending RX frames
> > > prior to a switch. It remains to be seen experimentally whether or not
> > > using a workqueue for RX processing would affect the time to switch
> > > channels negatively but I doubt it would be significant. I hope to
> > > test that with ath9k_htc.
> > > 
> > > What about gigabit or 10 Gigabit Ethernet drivers ? Do they face any
> > > challenges which would yet need to be proven would not face issues
> > > when processing bottom halves in process context?
> > > 
> > > [1] http://lwn.net/Articles/239633/
> > > 
> > >   Luis
> > > --
> > > To unsubscribe from this list: send the line "unsubscribe netdev" in
> > > the body of a message to majordomo@vger.kernel.org
> > > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> > 
> > Why not use NAPI, which is soft irq? Almost all 1G and 10G drivers
> > use NAPI.
> > 
> > Process context is too slow.
> 
> Well, I'm hoping to prove the opposite. I'm working on some stuff that I
> plan to present at Linux Plumbers. I've been too distracted by other
> things, but hopefully I'll have some good numbers to present by then.
> 


That's great, does it keep the good properties of NAPI (irq disabling
and throttling?)



-- 

Re: Stop using tasklets for bottom halves

[Steven Rostedt]

On Tue, 2009-09-08 at 09:11 -0700, Stephen Hemminger wrote:

> > > Process context is too slow.
> > 
> > Well, I'm hoping to prove the opposite. I'm working on some stuff that I
> > plan to present at Linux Plumbers. I've been too distracted by other
> > things, but hopefully I'll have some good numbers to present by then.
> > 
> 
> 
> That's great, does it keep the good properties of NAPI (irq disabling
> and throttling?)

Not exactly sure what you mean by throttling, but I'm assuming it will.

As for irqs disabling, I'm trying to avoid doing that. Note, the device
will have its interrupts disabled, but not all other devices will.

-- Steve


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Re: Stop using tasklets for bottom halves

[Stephen Hemminger]

On Tue, 08 Sep 2009 12:40:23 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Tue, 2009-09-08 at 09:11 -0700, Stephen Hemminger wrote:
> 
> > > > Process context is too slow.
> > > 
> > > Well, I'm hoping to prove the opposite. I'm working on some stuff that I
> > > plan to present at Linux Plumbers. I've been too distracted by other
> > > things, but hopefully I'll have some good numbers to present by then.
> > > 
> > 
> > 
> > That's great, does it keep the good properties of NAPI (irq disabling
> > and throttling?)
> 
> Not exactly sure what you mean by throttling, but I'm assuming it will.
> 
> As for irqs disabling, I'm trying to avoid doing that. Note, the device
> will have its interrupts disabled, but not all other devices will.
> 
> -- Steve
> 
> 

The way NAPI works is that in irq routine, the device disables interrupts
then schedules processing packets, when processing is done irq's are re-enabled.
This means that if machine is being flooded, irq's stay off, and the packets
get discarded (because device hardware ring is full), rather than in software
(because software receive queue is full).


-- 

Re: Stop using tasklets for bottom halves

[Steven Rostedt]

[ added Thomas Gleixner to Cc]

On Tue, 2009-09-08 at 10:01 -0700, Stephen Hemminger wrote:
> On Tue, 08 Sep 2009 12:40:23 -0400
> Steven Rostedt <rostedt-nx8X9YLhiw1AfugRpC6u6w@public.gmane.org> wrote:
> 
> > On Tue, 2009-09-08 at 09:11 -0700, Stephen Hemminger wrote:
> > 
> > > > > Process context is too slow.
> > > > 
> > > > Well, I'm hoping to prove the opposite. I'm working on some stuff that I
> > > > plan to present at Linux Plumbers. I've been too distracted by other
> > > > things, but hopefully I'll have some good numbers to present by then.
> > > > 
> > > 
> > > 
> > > That's great, does it keep the good properties of NAPI (irq disabling
> > > and throttling?)
> > 
> > Not exactly sure what you mean by throttling, but I'm assuming it will.
> > 
> > As for irqs disabling, I'm trying to avoid doing that. Note, the device
> > will have its interrupts disabled, but not all other devices will.
> > 
> > -- Steve
> > 
> > 
> 
> The way NAPI works is that in irq routine, the device disables interrupts
> then schedules processing packets, when processing is done irq's are re-enabled.
> This means that if machine is being flooded, irq's stay off, and the packets
> get discarded (because device hardware ring is full), rather than in software
> (because software receive queue is full).

That sounds exactly like what threaded IRQs will do. When an interrupt
comes in, the device driver will disable the device interrupts, and then
the device irq thread handler is awoken.

The device irq handler will handle all the packets. If new packets come
in, and the hardware ring buffer is full, those packets will be dropped.
When the irq handler thread is done processing all pending packets, it
will re-enable the device's interrupts and go to sleep.

Yeah, looking at the NAPI code, it does seem to follow what threaded
interrupts do.

-- Steve


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Re: Stop using tasklets for bottom halves

[Stephen Hemminger]

On Mon, 07 Sep 2009 22:17:34 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Mon, 2009-09-07 at 17:14 -0700, Stephen Hemminger wrote:
> > On Mon, 7 Sep 2009 15:58:50 -0700
> > "Luis R. Rodriguez" <mcgrof@gmail.com> wrote:
> > 
> > > A while ago I had read about an effort to consider removing tasklets
> > > [1] or at least trying to not use them. I'm unaware of the progress in
> > > this respect but since reading that article have always tried to
> > > evaluate whether or not we need tasklets on wireless drivers. I have
> > > also wondered whether work in irq context in other parts of the kernel
> > > can be moved to process context, a curious example being timers. I'll
> > > personally be trying to using only process context on bottom halves on
> > > future drivers but I figured it may be a good time to ask how serious
> > > was avoiding tasklets or using wrappers in the future to avoid irq
> > > context is or is it advised. Do we have a general agreement this is a
> > > good step forward to take? Has anyone made tests or changes on a
> > > specific driver from irq context to process context and proven there
> > > are no significant advantages of using irq context where you would
> > > have expected it?
> > > 
> > > Wireless in particular should IMHO not require taskets for anything
> > > time sensitive that I can think about except perhaps changing channels
> > > quickly and to do that appropriately also process pending RX frames
> > > prior to a switch. It remains to be seen experimentally whether or not
> > > using a workqueue for RX processing would affect the time to switch
> > > channels negatively but I doubt it would be significant. I hope to
> > > test that with ath9k_htc.
> > > 
> > > What about gigabit or 10 Gigabit Ethernet drivers ? Do they face any
> > > challenges which would yet need to be proven would not face issues
> > > when processing bottom halves in process context?
> > > 
> > > [1] http://lwn.net/Articles/239633/
> > > 
> > >   Luis
> > > --
> > > To unsubscribe from this list: send the line "unsubscribe netdev" in
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> > > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> > 
> > Why not use NAPI, which is soft irq? Almost all 1G and 10G drivers
> > use NAPI.
> > 
> > Process context is too slow.
> 
> Well, I'm hoping to prove the opposite. I'm working on some stuff that I
> plan to present at Linux Plumbers. I've been too distracted by other
> things, but hopefully I'll have some good numbers to present by then.
> 
> -- Steve
> 
> 

A good performance test is changing the behaviour of loopback
device and running lmbench.  This checks overhead without the specter
of real hardware.

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