[RFC] dccp ccid-3: High-res or low-res timers?

[RFC] dccp ccid-3: High-res or low-res timers?

[Gerrit Renker]

I would appreciate some advice and insights regarding the use of
high-resolution timers within a transport protocol, specifically
DCCP with CCID-3 (RFC 5348).

Currently the implementation is in a limbo of high-resolution and
low-resolution code. It is not good, neither here nor there, so
I would like to work on making the interface consistent.

After thinking this through I encountered a number of points
which made me question whether high-resolution timers will lead to
better performance and a cleaner interface.

I'd appreciate comments and input on this, the points are below.

1. Handling unavoidable waiting times
-------------------------------------
 One can not expect that, if the scheduling clock says 'send in x
 microseconds', a packet will indeed leave after x microseconds; due to
 waiting times. An example is in net/dccp/timer.c, when the socket is
 currently locked - we wait for a "small" amount of time:

	bh_lock_sock(sk);
	if (sock_owned_by_user(sk))
		sk_reset_timer(sk, &dp->dccps_xmit_timer, jiffies + 1);
	else
		dccp_write_xmit(sk, 0);
	bh_unlock_sock(sk);


2. Dependency on high-resolution timers
---------------------------------------
 Committing the CCID-3/CCID-4 implementations to using high-resolution
 timers means that the modules can not be built/loaded when the kernel
 does not offer sufficient resolution.

 This has recently made it hard for someone using CCID-3 to find out
 why DCCP would not run, where the cause was that high-resolution timers
 were not enabled in the kernel.


3. Noise in the output
----------------------
When tracking the speed of a car every 10 seconds, there is a lot of variation
in the values, due to stopping at traffic lights, accelerating etc. But when
considering a larger timescale, one can say that the average speed from city
A to city B was xx mph, since the whole journey took 2.5 hours.

The same can currently be observed with X_recv - there is one commit which 
tries to make X_recv as fine-grained as possible, it is labelled "dccp ccid-3:
Update the computation of X_recv",
http://eden-feed.erg.abdn.ac.uk/cgi-bin/gitweb.cgi?p‹cp_exp.git;a=commitdiff;h-0b687025494e5d8918ffcc7029d793390835cc

The result is that X_recv now shows much wider variation, on a small timescale
there is a lot happening. It can best be seen by plotting the X_recv using
dccp_probe. Without this commit the graphs are much 'quieter' and just show
the long-term average.

In TCP Westwood for instance a low-pass filter is used to filter out the 
high-frequency changes in the measurements of the Ack Rate:

"TCP Westwood: Bandwidth Estimation for Enhanced Transport over Wireless Links"
Mobicom 2001
http://www.cs.ucla.edu/NRL/hpi/tcpw/tcpw_papers/2001-mobicom-0.pdf

I'd appreciate opinions on this, as I think 

With regard to CCID-3, it also seems to be be better to revert the above
commit and just use long-term averages.


4. Not sure using high-resolution is the answer
-----------------------------------------------
While a fine-grained timer resolution may be desirable, it is not
necessarily a must. The implementation of rate-based pacing in TCP
(http://www.isi.edu/~johnh/PAPERS/Visweswaraiah97b.html) for instance
also used low(er) resolution timers and it worked.

The RFC for CCID-3 (http://www.rfc-archive.org/getrfc.php?rfcS48) also
does not high-resolution; it supports coarse-grained timestamps (section
6.3 and RFC 4342) and discusses implementation issues when using a 
lower resolution (section 8.3).

The counter-argument could be that CCID-3 is a transport protocol with a
built-in Token Bucket Filter so that similar considerations apply as for
the Qdisc API (net/sched/sch_api.c).

Summing up, I have doubts that basing CCID-3 will bring advantages and
would much rather go the other way and (consistently) use lower resolution.

Thoughts?
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Re: [RFC] dccp ccid-3: High-res or low-res timers?

[Ian McDonald]

OK. I'll add a few comments even though I'm a little bit rusty...

I was previously an advocate of low resolution timers and then use
bursts as needed to achieve the average rate as specified in RFC3448.

The reasoning for this was very much as you discuss in point 1 - that
you achieve less than the desired rate with high resolution timers as
you will never get exactly to transmit at the time you require (unless
you have a "hard" realtime system with desired accuracy) - so any
delay will slow down your transmit rate, and that high resolution
timers may not be available on all architectures. I also had a third
reason - overhead - if you're interrupting other tasks and having to
do a context switch many, many times a second surely that isn't so
good?

However RFC 5348 changes this as this clause is added to 4.6:
  To limit burstiness, a TFRC implementation MUST prevent bursts of
   arbitrary size.  This limit MUST be less than or equal to one round-
   trip time's worth of packets.  A TFRC implementation MAY limit bursts
   to less than a round-trip time's worth of packets

and this is further explained in section 8.3 and the downside - that
you can't send big bursts so you can't get the full calculated rate.

The RFC uses an example of 1 msec scheduling and 0.1 msec RTT. However
what would be worse is devices on a LAN with 10 msec timer - e.g. two
embedded devices at home - I haven't done the maths but I think the
rate achievable would be quite low.

One thing that I think we do need to be careful about though is
assuming that we should be trying to get very high speed transfer -
DCCP is not what we would layer a file serving protocol on top of....
(some have argued you shouldn't even use TCP for this on a LAN...)

Thinking laterally there is another possible solution - something I
used way back in the 80s for another project - build your own
scheduler! We could set a high resolution timer to tick every 0.1 msec
and then use the coarse grained algorithm at that point....

This is a hack to some degree and I can imagine David Miller
suggesting that it is more a protocol issue... The other thing is that
if we did this we would have to only do it when we actually need and
use higher granularity at other times or else the Powertop people may
not be so happy.

Anyway - something to think about. I've also added the IETF list as
well in case people there have the answers.

Regards

Ian

On Sat, Nov 15, 2008 at 11:50 PM, Gerrit Renker <gerrit@erg.abdn.ac.uk> wrote:
>
> I would appreciate some advice and insights regarding the use of
> high-resolution timers within a transport protocol, specifically
> DCCP with CCID-3 (RFC 5348).
>
> Currently the implementation is in a limbo of high-resolution and
> low-resolution code. It is not good, neither here nor there, so
> I would like to work on making the interface consistent.
>
> After thinking this through I encountered a number of points
> which made me question whether high-resolution timers will lead to
> better performance and a cleaner interface.
>
> I'd appreciate comments and input on this, the points are below.
>
> 1. Handling unavoidable waiting times
> -------------------------------------
>  One can not expect that, if the scheduling clock says 'send in x
>  microseconds', a packet will indeed leave after x microseconds; due to
>  waiting times. An example is in net/dccp/timer.c, when the socket is
>  currently locked - we wait for a "small" amount of time:
>
>        bh_lock_sock(sk);
>        if (sock_owned_by_user(sk))
>                sk_reset_timer(sk, &dp->dccps_xmit_timer, jiffies + 1);
>        else
>                dccp_write_xmit(sk, 0);
>        bh_unlock_sock(sk);
>
>
> 2. Dependency on high-resolution timers
> ---------------------------------------
>  Committing the CCID-3/CCID-4 implementations to using high-resolution
>  timers means that the modules can not be built/loaded when the kernel
>  does not offer sufficient resolution.
>
>  This has recently made it hard for someone using CCID-3 to find out
>  why DCCP would not run, where the cause was that high-resolution timers
>  were not enabled in the kernel.
>
>
> 3. Noise in the output
> ----------------------
> When tracking the speed of a car every 10 seconds, there is a lot of variation
> in the values, due to stopping at traffic lights, accelerating etc. But when
> considering a larger timescale, one can say that the average speed from city
> A to city B was xx mph, since the whole journey took 2.5 hours.
>
> The same can currently be observed with X_recv - there is one commit which
> tries to make X_recv as fine-grained as possible, it is labelled "dccp ccid-3:
> Update the computation of X_recv",
> http://eden-feed.erg.abdn.ac.uk/cgi-bin/gitweb.cgi?pÜcp_exp.git;a=commitdiff;h-0b687025494e5d8918ffcc7029d793390835cc
>
> The result is that X_recv now shows much wider variation, on a small timescale
> there is a lot happening. It can best be seen by plotting the X_recv using
> dccp_probe. Without this commit the graphs are much 'quieter' and just show
> the long-term average.
>
> In TCP Westwood for instance a low-pass filter is used to filter out the
> high-frequency changes in the measurements of the Ack Rate:
>
> "TCP Westwood: Bandwidth Estimation for Enhanced Transport over Wireless Links"
> Mobicom 2001
> http://www.cs.ucla.edu/NRL/hpi/tcpw/tcpw_papers/2001-mobicom-0.pdf
>
> I'd appreciate opinions on this, as I think
>
> With regard to CCID-3, it also seems to be be better to revert the above
> commit and just use long-term averages.
>
>
> 4. Not sure using high-resolution is the answer
> -----------------------------------------------
> While a fine-grained timer resolution may be desirable, it is not
> necessarily a must. The implementation of rate-based pacing in TCP
> (http://www.isi.edu/~johnh/PAPERS/Visweswaraiah97b.html) for instance
> also used low(er) resolution timers and it worked.
>
> The RFC for CCID-3 (http://www.rfc-archive.org/getrfc.php?rfcS48) also
> does not high-resolution; it supports coarse-grained timestamps (section
> 6.3 and RFC 4342) and discusses implementation issues when using a
> lower resolution (section 8.3).
>
> The counter-argument could be that CCID-3 is a transport protocol with a
> built-in Token Bucket Filter so that similar considerations apply as for
> the Qdisc API (net/sched/sch_api.c).
>
> Summing up, I have doubts that basing CCID-3 will bring advantages and
> would much rather go the other way and (consistently) use lower resolution.
>
> Thoughts?



--
Web: http://wand.net.nz/~iam4/, http://www.jandi.co.nz
Blog: http://iansblog.jandi.co.nz

Re: [RFC] dccp ccid-3: High-res or low-res timers?

[David Miller]

From: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Date: Sat, 15 Nov 2008 11:50:42 +0100

> 1. Handling unavoidable waiting times
> -------------------------------------
>  One can not expect that, if the scheduling clock says 'send in x
>  microseconds', a packet will indeed leave after x microseconds; due to
>  waiting times. An example is in net/dccp/timer.c, when the socket is
>  currently locked - we wait for a "small" amount of time:
> 
> 	bh_lock_sock(sk);
> 	if (sock_owned_by_user(sk))
> 		sk_reset_timer(sk, &dp->dccps_xmit_timer, jiffies + 1);
> 	else
> 		dccp_write_xmit(sk, 0);
> 	bh_unlock_sock(sk);

This should not happen often enough to be statistically meaningful.

If it does we have serious lock contention and we should fix that.

> 2. Dependency on high-resolution timers
> ---------------------------------------
>  Committing the CCID-3/CCID-4 implementations to using high-resolution
>  timers means that the modules can not be built/loaded when the kernel
>  does not offer sufficient resolution.
> 
>  This has recently made it hard for someone using CCID-3 to find out
>  why DCCP would not run, where the cause was that high-resolution timers
>  were not enabled in the kernel.

This could be argued as a bug in the hrtimer interface.  What it should
do is let you use the interfaces always, and the kernel gives you as
fine a resolution as the current configuration supports.

> 3. Noise in the output
> ----------------------
> When tracking the speed of a car every 10 seconds, there is a lot of variation
> in the values, due to stopping at traffic lights, accelerating etc. But when
> considering a larger timescale, one can say that the average speed from city
> A to city B was xx mph, since the whole journey took 2.5 hours.

This argument is sound.

> 4. Not sure using high-resolution is the answer
> -----------------------------------------------
> While a fine-grained timer resolution may be desirable, it is not
> necessarily a must. The implementation of rate-based pacing in TCP
> (http://www.isi.edu/~johnh/PAPERS/Visweswaraiah97b.html) for instance
> also used low(er) resolution timers and it worked.
> 
> The RFC for CCID-3 (http://www.rfc-archive.org/getrfc.php?rfcS48) also
> does not high-resolution; it supports coarse-grained timestamps (section
> 6.3 and RFC 4342) and discusses implementation issues when using a 
> lower resolution (section 8.3).
> 
> The counter-argument could be that CCID-3 is a transport protocol with a
> built-in Token Bucket Filter so that similar considerations apply as for
> the Qdisc API (net/sched/sch_api.c).
> 
> Summing up, I have doubts that basing CCID-3 will bring advantages and
> would much rather go the other way and (consistently) use lower resolution.
> 
> Thoughts?

I wouldn't bother with high resolution timers, mostly because they are more
expensive and the benefit of them here is at best "unknown".

Re: [RFC] dccp ccid-3: High-res or low-res timers?

[Eddie Kohler]

Gerrit Renker wrote:
> I would appreciate some advice and insights regarding the use of
> high-resolution timers within a transport protocol, specifically
> DCCP with CCID-3 (RFC 5348).
> 
 > ...
 >
> Summing up, I have doubts that basing CCID-3 will bring advantages and
> would much rather go the other way and (consistently) use lower resolution.
> 
> Thoughts?

I agree.  If one way must be chosen, then choose lower resolution timers.

The biggest potential problem with lower-resolution timers is that a sender's 
rate might be limited, not by network characteristics, but by timer 
resolution.  But DCCP allows a fair amount of burstiness already.  And there 
may be ways to avoid rate limitation in common cases without resorting to 
hrtimers.  For example, a sending application could use a mixture of 
non-blocking system calls, allowing the sending application to "poke" the DCCP 
implementation on every scheduling.

At any rate, it seems worth trying.

Eddie

Re: [dccp] [RFC] dccp ccid-3: High-res or low-res timers? <cross

[Gorry Fairhurst]

Before I say more, I think I would like to prefix this with my own 
personal view:

Many applications using TFRC will not normally try to send at the 
maximum permitted rate (i.e. are data/application limited). I expect 
these to be typical of apps expected to use DCCP CCID-3. That is, TFRC 
for DCCP could be thought of as providing a congestion-responsive 
protocol that responds to congestion in a way that is not (much) worse 
than that of TCP - to me this does not imply a need for equality in 
terms of throughput with TCP (i.e. I think a good use of the algorithm 
is to prevent a TFRC application from sending more, or at least not much 
more than a TCP flow would have sent).


Now to the response:

TFRC has been put forward for a wide range of applications - from 
extremely low rate (e.g. vocoded VoIP) to high rate (>> Gbps), with LAN 
delays to wide-area/wireless environments with long RTTs. I suggest, at 
least in the immediate future, most common DCCP applications will 
operate at low (kbps) to medium rates (few Mbps). My suggestion is 
therefore aim for a good stack for low-medium rates, were course clock 
granuality may be OK, and which avoids some of the pitfalls identified 
by Gerrit's email. Does anyone have a different vision of the near future?

I read RFC 5348, s4.6 as saying you can send extra data after being data 
limited/idle, but shouldn't send long bursts  - my thinking was that 
this was intended to address an issue for long delay paths. Short RTT 
paths anyway allow the sender to rapidly grow the rate, and are not 
usually so constrained by TFRC (at least at typical application rates).


Gorry


Ian McDonald wrote:
> OK. I'll add a few comments even though I'm a little bit rusty...
> 
> I was previously an advocate of low resolution timers and then use
> bursts as needed to achieve the average rate as specified in RFC3448.
> 
> The reasoning for this was very much as you discuss in point 1 - that
> you achieve less than the desired rate with high resolution timers as
> you will never get exactly to transmit at the time you require (unless
> you have a "hard" realtime system with desired accuracy) - so any
> delay will slow down your transmit rate, and that high resolution
> timers may not be available on all architectures. I also had a third
> reason - overhead - if you're interrupting other tasks and having to
> do a context switch many, many times a second surely that isn't so
> good?
> 
> However RFC 5348 changes this as this clause is added to 4.6:
>   To limit burstiness, a TFRC implementation MUST prevent bursts of
>    arbitrary size.  This limit MUST be less than or equal to one round-
>    trip time's worth of packets.  A TFRC implementation MAY limit bursts
>    to less than a round-trip time's worth of packets
> 
> and this is further explained in section 8.3 and the downside - that
> you can't send big bursts so you can't get the full calculated rate.
> 
> The RFC uses an example of 1 msec scheduling and 0.1 msec RTT. However
> what would be worse is devices on a LAN with 10 msec timer - e.g. two
> embedded devices at home - I haven't done the maths but I think the
> rate achievable would be quite low.
> 
> One thing that I think we do need to be careful about though is
> assuming that we should be trying to get very high speed transfer -
> DCCP is not what we would layer a file serving protocol on top of....
> (some have argued you shouldn't even use TCP for this on a LAN...)
> 
> Thinking laterally there is another possible solution - something I
> used way back in the 80s for another project - build your own
> scheduler! We could set a high resolution timer to tick every 0.1 msec
> and then use the coarse grained algorithm at that point....
> 
> This is a hack to some degree and I can imagine David Miller
> suggesting that it is more a protocol issue... The other thing is that
> if we did this we would have to only do it when we actually need and
> use higher granularity at other times or else the Powertop people may
> not be so happy.
> 
> Anyway - something to think about. I've also added the IETF list as
> well in case people there have the answers.
> 
> Regards
> 
> Ian
> 
> On Sat, Nov 15, 2008 at 11:50 PM, Gerrit Renker <gerrit@erg.abdn.ac.uk> wrote:
>> I would appreciate some advice and insights regarding the use of
>> high-resolution timers within a transport protocol, specifically
>> DCCP with CCID-3 (RFC 5348).
>>
>> Currently the implementation is in a limbo of high-resolution and
>> low-resolution code. It is not good, neither here nor there, so
>> I would like to work on making the interface consistent.
>>
>> After thinking this through I encountered a number of points
>> which made me question whether high-resolution timers will lead to
>> better performance and a cleaner interface.
>>
>> I'd appreciate comments and input on this, the points are below.
>>
>> 1. Handling unavoidable waiting times
>> -------------------------------------
>>  One can not expect that, if the scheduling clock says 'send in x
>>  microseconds', a packet will indeed leave after x microseconds; due to
>>  waiting times. An example is in net/dccp/timer.c, when the socket is
>>  currently locked - we wait for a "small" amount of time:
>>
>>        bh_lock_sock(sk);
>>        if (sock_owned_by_user(sk))
>>                sk_reset_timer(sk, &dp->dccps_xmit_timer, jiffies + 1);
>>        else
>>                dccp_write_xmit(sk, 0);
>>        bh_unlock_sock(sk);
>>
>>
>> 2. Dependency on high-resolution timers
>> ---------------------------------------
>>  Committing the CCID-3/CCID-4 implementations to using high-resolution
>>  timers means that the modules can not be built/loaded when the kernel
>>  does not offer sufficient resolution.
>>
>>  This has recently made it hard for someone using CCID-3 to find out
>>  why DCCP would not run, where the cause was that high-resolution timers
>>  were not enabled in the kernel.
>>
>>
>> 3. Noise in the output
>> ----------------------
>> When tracking the speed of a car every 10 seconds, there is a lot of variation
>> in the values, due to stopping at traffic lights, accelerating etc. But when
>> considering a larger timescale, one can say that the average speed from city
>> A to city B was xx mph, since the whole journey took 2.5 hours.
>>
>> The same can currently be observed with X_recv - there is one commit which
>> tries to make X_recv as fine-grained as possible, it is labelled "dccp ccid-3:
>> Update the computation of X_recv",
>> http://eden-feed.erg.abdn.ac.uk/cgi-bin/gitweb.cgi?pÜcp_exp.git;a=commitdiff;h-0b687025494e5d8918ffcc7029d793390835cc
>>
>> The result is that X_recv now shows much wider variation, on a small timescale
>> there is a lot happening. It can best be seen by plotting the X_recv using
>> dccp_probe. Without this commit the graphs are much 'quieter' and just show
>> the long-term average.
>>
>> In TCP Westwood for instance a low-pass filter is used to filter out the
>> high-frequency changes in the measurements of the Ack Rate:
>>
>> "TCP Westwood: Bandwidth Estimation for Enhanced Transport over Wireless Links"
>> Mobicom 2001
>> http://www.cs.ucla.edu/NRL/hpi/tcpw/tcpw_papers/2001-mobicom-0.pdf
>>
>> I'd appreciate opinions on this, as I think
>>
>> With regard to CCID-3, it also seems to be be better to revert the above
>> commit and just use long-term averages.
>>
>>
>> 4. Not sure using high-resolution is the answer
>> -----------------------------------------------
>> While a fine-grained timer resolution may be desirable, it is not
>> necessarily a must. The implementation of rate-based pacing in TCP
>> (http://www.isi.edu/~johnh/PAPERS/Visweswaraiah97b.html) for instance
>> also used low(er) resolution timers and it worked.
>>
>> The RFC for CCID-3 (http://www.rfc-archive.org/getrfc.php?rfcS48) also
>> does not high-resolution; it supports coarse-grained timestamps (section
>> 6.3 and RFC 4342) and discusses implementation issues when using a
>> lower resolution (section 8.3).
>>
>> The counter-argument could be that CCID-3 is a transport protocol with a
>> built-in Token Bucket Filter so that similar considerations apply as for
>> the Qdisc API (net/sched/sch_api.c).
>>
>> Summing up, I have doubts that basing CCID-3 will bring advantages and
>> would much rather go the other way and (consistently) use lower resolution.
>>
>> Thoughts?
> 
> 
> 
> --
> Web: http://wand.net.nz/~iam4/, http://www.jandi.co.nz
> Blog: http://iansblog.jandi.co.nz
> 
> 

Re: [RFC] dccp ccid-3: High-res or low-res timers?

[Gerrit Renker]

Dave, -
| > Summing up, I have doubts that basing CCID-3 will bring advantages and
| > would much rather go the other way and (consistently) use lower resolution.
| > 
| > Thoughts?
| 
| I wouldn't bother with high resolution timers, mostly because they are more
| expensive and the benefit of them here is at best "unknown".
| 
Thanks a lot for the reply. I am glad for this suggestion, since it will
allow simpler code while at the same time still supporting a basic,
working implementation of CCID-3.

If people want to experiment with higher-resolution timers (as per other
replies), this can be put into an experimental subtree on 
eden-feed.erg.abdn.ac.uk.

If it is okay with Leandro and Tommi, it would finally also make it
possible to sort out consolidating the CCID-3/CCID-4 code.

Thanks again,
Gerrit

Re: [RFC] dccp ccid-3: High-res or low-res timers?

[Gerrit Renker]

Ian, -
| However RFC 5348 changes this as this clause is added to 4.6:
|   To limit burstiness, a TFRC implementation MUST prevent bursts of
|    arbitrary size.  This limit MUST be less than or equal to one round-
|    trip time's worth of packets.  A TFRC implementation MAY limit bursts
|    to less than a round-trip time's worth of packets
| 
| and this is further explained in section 8.3 and the downside - that
| you can't send big bursts so you can't get the full calculated rate.
| 
| The RFC uses an example of 1 msec scheduling and 0.1 msec RTT. However
| what would be worse is devices on a LAN with 10 msec timer - e.g. two
| embedded devices at home - I haven't done the maths but I think the
| rate achievable would be quite low.
If we use lower-resolution timers I think there should be a
recommendation (in the Kconfig menu for instance) not to use
low HZ values.

Previously this was done as a build warning, but it is annoying if
people do an allmodconfig and are not otherwise interested in DCCP.

| 
| One thing that I think we do need to be careful about though is
| assuming that we should be trying to get very high speed transfer -
| DCCP is not what we would layer a file serving protocol on top of....
| (some have argued you shouldn't even use TCP for this on a LAN...)
| 
This agrees with Gorry's reply and is an important point, since low
RTTs will be the rule when people use Gbit ethernet or loopback.

CCID-4 has a speed limiter of limiting the speed to 100 packets per
second, at Ethernet MTU this is still around 1 Mbps.

So the problem is that the parameters will suggest very high speeds,
while CCID-3 in fact targets lower speed ranges.

Do you think we could live with clamping the RTT to some sensible
minimum, since on a local LAN the use of congestion control is
questionable? I was thinking in the order of 0.5 ... 1msec.

I believe that with some sensible engineering and a suitable algorithm
it is possible to get good performance out of CCID-3 without resorting
to high-resolution timers, i.e. I think that your earlier emails
were right.

I have been looking at the jiffy-based TCP RTT estimator in
net/ipv4/tcp_input.c a lot. It is an excellent example that even with
low-resolution timers a good algorithm can make a lot of difference.
In tests it worked so well that this algorithm has been ported to 
replace the current CCID-2 RTT estimator.

| Thinking laterally there is another possible solution - something I
| used way back in the 80s for another project - build your own
| scheduler! We could set a high resolution timer to tick every 0.1 msec
| and then use the coarse grained algorithm at that point....
| 
So we have three possible options - timer-based (low/high), and your
suggestion above. We can keep these variants open by spawning an
experimental subtree which provides an alternative implementation, so
that people could explore alternative algorithms, compare and send patches.

For production use the low-resolution variant is the simplest and less
expensive option, and it is good that there is consensus about it.

In a discussion about two years ago there was another
idea, doing away with the nofeedback timer, by checking the nofeedback
time at the instant a packet is sent.

Gerrit

Re: [RFC] dccp ccid-3: High-res or low-res timers?

[Ian McDonald]

On Tue, Nov 18, 2008 at 7:14 PM, Gerrit Renker <gerrit@erg.abdn.ac.uk> wrote:
> | The RFC uses an example of 1 msec scheduling and 0.1 msec RTT. However
> | what would be worse is devices on a LAN with 10 msec timer - e.g. two
> | embedded devices at home - I haven't done the maths but I think the
> | rate achievable would be quite low.
> If we use lower-resolution timers I think there should be a
> recommendation (in the Kconfig menu for instance) not to use
> low HZ values.
>
> Previously this was done as a build warning, but it is annoying if
> people do an allmodconfig and are not otherwise interested in DCCP.
>
Agree

> Do you think we could live with clamping the RTT to some sensible
> minimum, since on a local LAN the use of congestion control is
> questionable? I was thinking in the order of 0.5 ... 1msec.
>
I think that is a good idea - if 1 msec, and HZ = 1000 then we
wouldn't lose any transmission capability.

> | Thinking laterally there is another possible solution - something I
> | used way back in the 80s for another project - build your own
> | scheduler! We could set a high resolution timer to tick every 0.1 msec
> | and then use the coarse grained algorithm at that point....
> |
> So we have three possible options - timer-based (low/high), and your
> suggestion above. We can keep these variants open by spawning an
> experimental subtree which provides an alternative implementation, so
> that people could explore alternative algorithms, compare and send patches.
>
> For production use the low-resolution variant is the simplest and less
> expensive option, and it is good that there is consensus about it.
>
Yes - and with your RTT clamping then no need to do my idea around scheduler.

> In a discussion about two years ago there was another
> idea, doing away with the nofeedback timer, by checking the nofeedback
> time at the instant a packet is sent.
>
I think this is useful as reduces the amount of timers going off,
which reduces system load.

-- 
Web: http://wand.net.nz/~iam4/, http://www.jandi.co.nz
Blog: http://iansblog.jandi.co.nz

Re: [RFC] dccp ccid-3: High-res or low-res timers?

[Gerrit Renker]

Thanks Ian for the comments and suggestions, which is input for further
work. Hope to start this poco a poco soon.

| > Do you think we could live with clamping the RTT to some sensible
| > minimum, since on a local LAN the use of congestion control is
| > questionable? I was thinking in the order of 0.5 ... 1msec.
| >
| I think that is a good idea - if 1 msec, and HZ = 1000 then we
| wouldn't lose any transmission capability.
| 
Probably it will require a bit of testing, it might be that some
fine-tuning is needed since a lower RTT also means a lower maximum
throughput -- but maybe that limit is fully sufficient for the target
operational range of CCID-3 (streaming, not bulk data transfer).

Gerrit