Hi,
Juga and pastly have been working hard on sbws.
Sbws' results are now similar to torflow's results: https://trac.torproject.org/projects/tor/attachment/ticket/27135/20180826_08...
Now that sbws is close to torflow, we want some feedback on its design. We’ll work on the design at the tor meeting in September.
Please feel free to give feedback by email, or on the tickets:
What happens when sbws doesn't match torflow?
https://trac.torproject.org/projects/tor/ticket/27339
We suggest this rule:
If an sbws deployment is within X% of an existing bandwidth authority, sbws is ok. (The total consensus weights of the existing bandwidth authorities are within 25% - 50% of each other, see #25459.)
How long should sbws keep relay bandwidths?
https://trac.torproject.org/projects/tor/ticket/27338
Torflow uses the latest self-reported relay observed bandwidth and bandwidth rate.
Torflow uses a complex feedback loop for measured bandwidths. We think sbws can use a simple average or exponentially decaying weighted average.
How should we scale sbws consensus weights?
https://trac.torproject.org/projects/tor/ticket/27340
If sbws' total consensus weight is different to torflow's total consensus weight, how should we scale sbws?
(The weights might differ because the measurement method is different, or because scanners and servers are in different locations.)
In the bandwidth file spec, we suggest linear scaling.
How should we round sbws consensus weights?
https://trac.torproject.org/projects/tor/ticket/27337
Torflow currently rounds to 3 significant figures (which is a maximum of 0.5%). But I suggest 2 significant figures for sbws (or max 5%), because: - tor has a daily usage cycle that varies by 10% - 20% - existing bandwidth authorities vary by 25% - 50%
Proposal 276 contains a slightly more complicated rounding algorithm, which we may want to implement in sbws or in tor:
https://gitweb.torproject.org/torspec.git/tree/proposals/276-lower-bw-granul...
Does sbws need a maximum consensus weight fraction?
https://trac.torproject.org/projects/tor/ticket/27336
Torflow uses 5%, but I suggest 1%, because the largest relay right now is only 0.5%.
T
-- teor
Please reply @torproject.org New subkeys 1 July 2018 PGP C855 6CED 5D90 A0C5 29F6 4D43 450C BA7F 968F 094B ----------------------------------------------------------------------
teor:
Hi,
Juga and pastly have been working hard on sbws.
Sbws' results are now similar to torflow's results: https://trac.torproject.org/projects/tor/attachment/ticket/27135/20180826_08...
Congratulations, Juga and pastly!
Now that sbws is close to torflow, we want some feedback on its design. We’ll work on the design at the tor meeting in September.
Please feel free to give feedback by email, or on the tickets:
What happens when sbws doesn't match torflow?
https://trac.torproject.org/projects/tor/ticket/27339
We suggest this rule:
If an sbws deployment is within X% of an existing bandwidth authority, sbws is ok. (The total consensus weights of the existing bandwidth authorities are within 25% - 50% of each other, see #25459.)
I would like an additional criteria for when we finally replace torflow with sbws.
Ideally, I would like us to perform A/B experiments to ensure that our performance metrics do not degrade in terms of average *or* quartile range/performance variance. (Ie: alternate torflow results for a week vs sbws for a week, and repeat for a few weeks). I realize this might be complicated for dirauth operators, though. Can we make it easier somehow, so that it is easy to switch which result files they are voting with?
If we can't do this, at minimum, we should definitely watch the change in our average and quartile variance performance metrics when we first switch to sbws.
Additionally, if we ever change how sbws behaves to be different than torflow, I would like sbws to have a well-defined load balancing equilibrium goal, and I would like us to not change this load balancing equilibrium goal unless we perform A/B testing and compare the average and variance of our performance metrics.
I'll explain what I mean by "load balancing equilibrium goal" below, when I try to explain the PID mechanism again.
How long should sbws keep relay bandwidths?
https://trac.torproject.org/projects/tor/ticket/27338
Torflow uses the latest self-reported relay observed bandwidth and bandwidth rate.
Torflow uses a complex feedback loop for measured bandwidths. We think sbws can use a simple average or exponentially decaying weighted average.
As I said in https://lists.torproject.org/pipermail/tor-dev/2017-December/012714.html, this feedback loop is disabled. I know you don't believe that the bandwidth auth spec is accurate, but I'm telling you it is. There's just a lot going on there because the bwauths have required a long history of experimentation to get to where they are now, just as sbws is now encountering with trying to make various measurement and scaling decisions. (As you A/B test ways to improve performance on the live network, you tend to accumulate a lot of options for different ways of doing things).
The point of the PID control stuff was to formalize the type of load balancing equilibrium goal that the bandwidth auths are using, and to experiment with convergence on a specific target load balancing equilibrium point (where that target equilibrium point is "all relays have the same spare capacity for one additional client stream"). The problem was that when you only use this criteria, faster relays run out of CPU, memory, or sockets before this criteria was satisfied for them. Hence all of the circuit failure reason statistics in the code base (to try to back off on PID control if we hit a different limiting factor other than bandwidth).
Unfortunately, Tor does not provide enough error code feedback to reliably determine if a relay is low on memory, sockets, or CPU. Funding ended for the bandwidths auths before we could implement proper overload error feedback in Tor, and we got funding for me to work on Tor Browser instead.
With the parameters in the current consensus (currently bwauthpid=1, and no others), the PID control is operating as only "Proportional control": https://gitweb.torproject.org/torflow.git/tree/NetworkScanners/BwAuthority/R... (The default values for K_i and K_d are 0, as per Section 3.6 of the spec).
In section 3.1 of the spec, I have a proof that using "Proportional control" (ie PID control with no I or D) is equivalent to what we were doing in Section 2.2. This means that Section 2.2 does describe what we are doing now: https://gitweb.torproject.org/torflow.git/tree/NetworkScanners/BwAuthority/R... https://gitweb.torproject.org/torflow.git/tree/NetworkScanners/BwAuthority/R...
I left the PID code itself enabled (but in "Proportional-only" mode) because it is cleaner, and it makes it formally clear that the bandwidth authorities are actually measuring the difference in the ability of relays to carry additional client traffic, and correcting for that difference by adjusting weights in proportion to that difference. I naively assumed that eventually Tor would get funding to implement better feedback for CPU, memory, and socket overload. That was almost 10 years ago.
(Incidentally, the trickiest non-bandwidth overload condition to report was memory shortage -- a problem that effectively goes away in a datagram Tor world with bounded queue lengths.. In fact, CPU limits could also be implemented as a congestion drop condition in a datagram scenario, and measured indirectly via relay throughput, leaving only socket limits.)
I'm glad that we are exploring load balancing again, and with a modern, simpler, and well-tested code base. That's all good. But as you make choices about how to load balance, please have a specific goal as to what target load balancing equilibrium point you're actually going for.
(The reason why I did not consider raw measured stream bandwidth to be a valid equilibrium is because it is *not* measuring the total capacity of a relay, and it does not have an equilibrium point in terms of expected client performance or overall load balancing. In order to use raw measurements directly as a load balancing equilibrium point, you actually need to measure total relay throughput in some way, such as Peerflow did).
How should we scale sbws consensus weights?
https://trac.torproject.org/projects/tor/ticket/27340
If sbws' total consensus weight is different to torflow's total consensus weight, how should we scale sbws?
(The weights might differ because the measurement method is different, or because scanners and servers are in different locations.)
In the bandwidth file spec, we suggest linear scaling.
This seems reasonable.
I am wary of the idea of trying to use some kind of ideal distribution for relays, that you mention in #27135. That is not something that you can enforce in measurement without causing performance variance to suffer tremendously. It can only be enforced by a consensus cutoff threshold and/or relay operator incentive mechanisms.
I believe quite strongly that even if the Tor network gets faster on average, if this comes at the cost of increased performance variance, user experience and perceived speed of Tor will be much worse. There's nothing more annoying than a system that is *usually* fast enough to do what you need it to do, but fails to be fast enough for that activity at unpredictable times.
How should we round sbws consensus weights?
https://trac.torproject.org/projects/tor/ticket/27337
Torflow currently rounds to 3 significant figures (which is a maximum of 0.5%). But I suggest 2 significant figures for sbws (or max 5%), because:
- tor has a daily usage cycle that varies by 10% - 20%
- existing bandwidth authorities vary by 25% - 50%
Proposal 276 contains a slightly more complicated rounding algorithm, which we may want to implement in sbws or in tor:
https://gitweb.torproject.org/torspec.git/tree/proposals/276-lower-bw-granul...
If we can measure relays frequently enough such that we can accurately report the effects of Tor's daily usage cycle and adjust our weights accordingly, then I think that retaining the ability to represent this variance is worth the overhead.
Again, this comes back to my belief that performance variance is actually the major performance problem facing Tor right now.
On the other hand, if we cannot measure accurately or often enough for this to matter, then it doesn't matter.
But a successor to sbws might, if we can manage to build one sooner than a decade from now, so it would be wise not to bake this sig fig limit into our actual consensus format.
Does sbws need a maximum consensus weight fraction?
https://trac.torproject.org/projects/tor/ticket/27336
Torflow uses 5%, but I suggest 1%, because the largest relay right now is only 0.5%.
Sounds reasonable.
If we ever get working multi-core crypto+networking, this number will change, though.
On 29 August 2018 at 16:11, Mike Perry mikeperry@torproject.org wrote:
Ideally, I would like us to perform A/B experiments to ensure that our performance metrics do not degrade in terms of average *or* quartile range/performance variance. (Ie: alternate torflow results for a week vs sbws for a week, and repeat for a few weeks). I realize this might be complicated for dirauth operators, though. Can we make it easier somehow, so that it is easy to switch which result files they are voting with?
Having both voting files means running both scanners at the same time. Depending on one's pipes, that might skew the results from the scanners.
-tom
Hi,
We have deployed sbws on one bandwidth authority (longclaw).
Here's a request for additional feedback, and a progress update:
Request for Feedback: Relay Bandwidth Self-Tests
Torflow and sbws use relays' self-reported observed bandwidths for load balancing. But relays can have really low bandwidths because they're new, or due to random path selection.
In torflow, relays can get stuck in a low-bandwidth partition. sbws doesn't have partitions. But in both systems, low bandwidths can cause inaccurate or unstable load balancing.
Since torflow and sbws need accurate self-reported relay bandwidths, some component of the Tor network needs to send enough bandwidth through every relay.
Here are our current choices:
Tor relays can do a regular bandwidth self-test, so that their first descriptor has an accurate bandwidth (up to some minimum). But the current self-test is too small, and buggy.
sbws already sends bandwidth to all relays to measure them. sbws gets accurate bandwidths for most relays within 2 weeks, but the fastest relays can take a month to ramp up. (sbws starts measuring at the median relay bandwidth, and can double every 5 days.)
Should we improve relay bandwidth self-tests? (#22453) Or should we rely on sbws to create the bandwidths it needs? What about test networks?
Should we make bandwidths grow faster in sbws? Or is a ramp-up period of 2-5 weeks fast enough?
(We won't modify and re-deploy torflow.)
Progress Update
On 30 Aug 2018, at 07:11, Mike Perry mikeperry@torproject.org wrote:
teor:
What happens when sbws doesn't match torflow?
https://trac.torproject.org/projects/tor/ticket/27339
We suggest this rule:
If an sbws deployment is within X% of an existing bandwidth authority, sbws is ok. (The total consensus weights of the existing bandwidth authorities are within 25% - 50% of each other, see #25459.)
We have successfully used this rule to discover and fix some bugs in sbws.
I would like an additional criteria for when we finally replace torflow with sbws.
Ideally, I would like us to perform A/B experiments to ensure that our performance metrics do not degrade in terms of average *or* quartile range/performance variance. (Ie: alternate torflow results for a week vs sbws for a week, and repeat for a few weeks). I realize this might be complicated for dirauth operators, though. Can we make it easier somehow, so that it is easy to switch which result files they are voting with?
We do not have the capacity to A/B test sbws and torflow. (As far as I understand, we don't have enough people, and we don't have enough servers.)
If we can't do this, at minimum, we should definitely watch the change in our average and quartile variance performance metrics when we first switch to sbws.
We deployed sbws on 1/6 bandwidth authorities, and the performance of the network has been stable: https://metrics.torproject.org/torperf.html?start=2018-01-21&end=2018-11...
(The drop in performance at the start of the year was due to extra network load.)
Additionally, if we ever change how sbws behaves to be different than torflow, I would like sbws to have a well-defined load balancing equilibrium goal, and I would like us to not change this load balancing equilibrium goal unless we perform A/B testing and compare the average and variance of our performance metrics.
I'll explain what I mean by "load balancing equilibrium goal" below, when I try to explain the PID mechanism again.
sbws has adopted Torflow's load-balancing equilibrium goal.
Our priority is transitioning away from Torflow successfully.
We've deferred changes to the load-balancing goal until a later sbws release. We may never make this change.
How long should sbws keep relay bandwidths?
https://trac.torproject.org/projects/tor/ticket/27338
Torflow uses the latest self-reported relay observed bandwidth and bandwidth rate.
Torflow uses a complex feedback loop for measured bandwidths. We think sbws can use a simple average or exponentially decaying weighted average.
As I said in https://lists.torproject.org/pipermail/tor-dev/2017-December/012714.html, this feedback loop is disabled. I know you don't believe that the bandwidth auth spec is accurate, but I'm telling you it is.
Improving bandwidth measurement has been one of the most difficult things I have done with Tor.
You're right: I don't know if the Torflow spec is accurate, because I often struggle to find the information I need in the spec.
That's not anyone's fault: it's a difficult and complex topic. But it does mean that I need your help to answer some questions about Torflow.
The point of the PID control stuff was to formalize the type of load balancing equilibrium goal that the bandwidth auths are using, and to experiment with convergence on a specific target load balancing equilibrium point (where that target equilibrium point is "all relays have the same spare capacity for one additional client stream"). The problem was that when you only use this criteria, faster relays run out of CPU, memory, or sockets before this criteria was satisfied for them. Hence all of the circuit failure reason statistics in the code base (to try to back off on PID control if we hit a different limiting factor other than bandwidth).
...
I'm glad that we are exploring load balancing again, and with a modern, simpler, and well-tested code base. That's all good. But as you make choices about how to load balance, please have a specific goal as to what target load balancing equilibrium point you're actually going for.
sbws has adopted Torflow's goals.
How should we scale sbws consensus weights?
https://trac.torproject.org/projects/tor/ticket/27340
If sbws' total consensus weight is different to torflow's total consensus weight, how should we scale sbws?
(The weights might differ because the measurement method is different, or because scanners and servers are in different locations.)
In the bandwidth file spec, we suggest linear scaling.
This seems reasonable.
Unfortunately, linear scaling did not work.
sbws now uses Torflow's scaling method, with relay observed bandwidths.
...
I believe quite strongly that even if the Tor network gets faster on average, if this comes at the cost of increased performance variance, user experience and perceived speed of Tor will be much worse. There's nothing more annoying than a system that is *usually* fast enough to do what you need it to do, but fails to be fast enough for that activity at unpredictable times.
I agree. And I'm usually using Tor in high-latency locations, so I see this variance every day.
How should we round sbws consensus weights?
https://trac.torproject.org/projects/tor/ticket/27337
Torflow currently rounds to 3 significant figures (which is a maximum of 0.5%). But I suggest 2 significant figures for sbws (or max 5%), because:
- tor has a daily usage cycle that varies by 10% - 20%
- existing bandwidth authorities vary by 25% - 50%
Proposal 276 contains a slightly more complicated rounding algorithm, which we may want to implement in sbws or in tor:
https://gitweb.torproject.org/torspec.git/tree/proposals/276-lower-bw-granul...
If we can measure relays frequently enough such that we can accurately report the effects of Tor's daily usage cycle and adjust our weights accordingly, then I think that retaining the ability to represent this variance is worth the overhead.
Again, this comes back to my belief that performance variance is actually the major performance problem facing Tor right now.
On the other hand, if we cannot measure accurately or often enough for this to matter, then it doesn't matter.
I don't believe sbws can measure relays fast enough for it to matter.
sbws was rounding to the nearest 1000 kilobytes on 1/6 authorities, with no discernible performance on the network. We've fixed this bug, and sbws will now round to 2 significant figures. (We haven't implemented the extra last-digit rounding in prop276.)
If sbws can measure fast enough in future, we can modify it to report more accurate bandwidths.
But a successor to sbws might, if we can manage to build one sooner than a decade from now, so it would be wise not to bake this sig fig limit into our actual consensus format.
Thanks, we won't modify tor to round bandwidths: that responsibility belongs in the bandwidth measurement code.
But tor's consensus diffs and compression benefit from rounded relay bandwidths, so any performance gain needs to be measured against an increase in consensus download sizes.
Does sbws need a maximum consensus weight fraction?
https://trac.torproject.org/projects/tor/ticket/27336
Torflow uses 5%, but I suggest 1%, because the largest relay right now is only 0.5%.
Sounds reasonable.
If we ever get working multi-core crypto+networking, this number will change, though.
We went with 5%, to match Torflow.
T
On Mon, Nov 19, 2018 at 7:36 AM teor teor@riseup.net wrote:
Hi,
We have deployed sbws on one bandwidth authority (longclaw).
Here's a request for additional feedback, and a progress update:
Request for Feedback: Relay Bandwidth Self-Tests
Torflow and sbws use relays' self-reported observed bandwidths for load balancing. But relays can have really low bandwidths because they're new, or due to random path selection.
In torflow, relays can get stuck in a low-bandwidth partition. sbws doesn't have partitions. But in both systems, low bandwidths can cause inaccurate or unstable load balancing.
Since torflow and sbws need accurate self-reported relay bandwidths, some component of the Tor network needs to send enough bandwidth through every relay.
Here are our current choices:
Tor relays can do a regular bandwidth self-test, so that their first descriptor has an accurate bandwidth (up to some minimum). But the current self-test is too small, and buggy.
sbws already sends bandwidth to all relays to measure them. sbws gets accurate bandwidths for most relays within 2 weeks, but the fastest relays can take a month to ramp up. (sbws starts measuring at the median relay bandwidth, and can double every 5 days.)
Should we improve relay bandwidth self-tests? (#22453) Or should we rely on sbws to create the bandwidths it needs? What about test networks?
Hi! I don't think I have the answers here, but maybe I can think aloud in a useful way.
From my point of view, either of these is a fine idea, if it works.
We could decide based on a lot of factors, like:
* Which one is easier to do? * Which creates the greater maintenance burden, moving forward? * Which is more robust if something breaks in the future? * Which consumes the most relay bandwidth? * Which requires SBWS to use the most bandwidth?
Maybe if we had those figured out, we'd have a better time deciding.
Should we make bandwidths grow faster in sbws? Or is a ramp-up period of 2-5 weeks fast enough?
I think that's fast enough, though I'm not sure. How does it compare with the current average torflow ramp-up time?
(We won't modify and re-deploy torflow.)
-
Mike Perry -
Nick Mathewson -
teor -
Tom Ritter