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Scalable but Wasteful, or why fast replication protocols are slow

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Re: Scalable but Wasteful, or why fast replication protocols are slow

#11

Post by luhn »

Honestly I think the answer is simpler: People don't need better algorithms. Paxos and Raft are generally used to build service discovery and node coordination, these are not demanding workloads and overwhelmingly read-heavy. Even the largest deployments can probably be serviced by a set of modestly-sized VMs. Paxos and Raft are well-understood algorithms with a choice of battle-tested implementations, why would anyone choose different?

The whole section on "bin-packing Paxos/Raft is more efficient" is strange, because people don't generally bin-pack Paxos/Raft—The bin-packing orchestrators are built off of Paxos/Raft!

Re: Scalable but Wasteful, or why fast replication protocols are slow

#12

Post by rubiquity »

luhn wrote: post #11

Honestly I think the answer is simpler: People don't need better algorithms. Paxos and Raft are generally used to build service discovery and node coordination, these are not demanding workloads and overwhelmingly read-heavy. Even the largest deployments can probably be serviced by a set of modestly-sized VMs. Paxos and Raft are well-understood algorithms with a choice of battle-tested implementations, why would anyo…

People would choose different because operating Raft/Paxos clusters at scale isn't fun. Also, scaling reads is the hard part! At this point the trade offs are pretty well known: you either send all reads to the leader and limit the leader's overall throughput or you pay the cost of reading from a majority.

Your opinions are very specific to using consensus in orchestrators and control planes but the overwhelming majority of writes and reads to a Paxos or Raft cluster are in much higher throughput and latency sensitive systems such as databases.

Indeed we don't need better consensus algorithms. We need to closely examine our problems and step down to weaker consistency models and protocols that aren't worse to operate than Raft/Paxos or harder for developers to program against where ever possible.

Re: Scalable but Wasteful, or why fast replication protocols are slow

#13

Post by nijave »

rdw wrote: post #8

The hypothesis in the article could be correct (that industry is not adopting new academic innovations because they fail in the real world). Based on my experience in this industry, though, it could just be that there isn't a super strong connection between academia and the people implementing these kinds of systems. I've had many conversations with my academically-minded friends where they're astonished that we have…

Hardware vs software is a big one. You have to hit a certain scale ("web scale") before improving software is cheaper than buying more hardware. Sometimes there are low-hanging fruits but I don't think "building a new distributed consensus system" is usually one of them

It seems these things usually get implemented when Google, Microsoft, Twitter, Facebook (big web company x) hit the problem and they each develop their own optimized solution before slowly open sourcing and products converge and mature to the point it's practical for other companies to adopt (I think container orchestration is a good example where Google, Facebook, Apple(?) each had container solutions long before K8s started exploding)

Re: Scalable but Wasteful, or why fast replication protocols are slow

#14

Post by eternalban »

nine_k wrote: post #6
eternalban wrote: post #4

Earlier quoted context omitted.

A leaderless protocol is more complex and requires greater IO and CPU resources per unit of work. This is the hidden cost. The question is if the cost is reasonably offset by higher performance (throughput) offered by leaderless consensus. OP is arguing that the benefit is not deemed worth the cost outside of academia and this is why e.g EPaxos is not adopted.

But doesn't a leaderless protocol also give you more resilience against failures? Or, in other words, can it be that the higher cost buys you not better throughput but faster reconciliation when connectivity is poor? Not in a data center but on a mobile network?

I had the same thought. My OP was summarizing the article for GP. My personal [sense] is that RAFT stole the limelight and EPaxos never got any blogsphere love. Few actually even knew about it.

Re: Scalable but Wasteful, or why fast replication protocols are slow

#15

Post by benlivengood »

LAC-Tech wrote: post #7

> The protocol presents a leader-less solution, where any node can become an opportunistic coordinator for an operation. Does leader = master here? My first reaction is that this is a multi-master system but I can't quite unpack "opportunistic coordinator".

For MultiPaxos the leader is sticky to avoid additional round-trips for each transaction. Basically until a configurable timeout all transactions are attempted at the last node to lead an accepted transaction, and only if that times out or fails is a new leader of a transaction proposed.

Re: Scalable but Wasteful, or why fast replication protocols are slow

#16

Post by Crash0v3rid3 »

nine_k wrote: post #6
eternalban wrote: post #4

Earlier quoted context omitted.

A leaderless protocol is more complex and requires greater IO and CPU resources per unit of work. This is the hidden cost. The question is if the cost is reasonably offset by higher performance (throughput) offered by leaderless consensus. OP is arguing that the benefit is not deemed worth the cost outside of academia and this is why e.g EPaxos is not adopted.

But doesn't a leaderless protocol also give you more resilience against failures? Or, in other words, can it be that the higher cost buys you not better throughput but faster reconciliation when connectivity is poor? Not in a data center but on a mobile network?

Putting my SRE hat on, I'd generally agree that a leaderless protocol would be the way to go to prevent failures but you will still have to consider the costs involved with running such a system.

Re: Scalable but Wasteful, or why fast replication protocols are slow

#17

Post by foota »

nine_k wrote: post #6
eternalban wrote: post #4

Earlier quoted context omitted.

A leaderless protocol is more complex and requires greater IO and CPU resources per unit of work. This is the hidden cost. The question is if the cost is reasonably offset by higher performance (throughput) offered by leaderless consensus. OP is arguing that the benefit is not deemed worth the cost outside of academia and this is why e.g EPaxos is not adopted.

But doesn't a leaderless protocol also give you more resilience against failures? Or, in other words, can it be that the higher cost buys you not better throughput but faster reconciliation when connectivity is poor? Not in a data center but on a mobile network?

Leader transfer is generally quick enough with Paxos in my experience.

Re: Scalable but Wasteful, or why fast replication protocols are slow

#18

Post by [deleted] »

rubiquity wrote: post #12
luhn wrote: post #11

Honestly I think the answer is simpler: People don't need better algorithms. Paxos and Raft are generally used to build service discovery and node coordination, these are not demanding workloads and overwhelmingly read-heavy. Even the largest deployments can probably be serviced by a set of modestly-sized VMs. Paxos and Raft are well-understood algorithms with a choice of battle-tested implementations, why would anyo…

People would choose different because operating Raft/Paxos clusters at scale isn't fun. Also, scaling reads is the hard part! At this point the trade offs are pretty well known: you either send all reads to the leader and limit the leader's overall throughput or you pay the cost of reading from a majority. Your opinions are very specific to using consensus in orchestrators and control planes but the overwhelming majo…

[deleted]

Re: Scalable but Wasteful, or why fast replication protocols are slow

#19

Post by LAC-Tech »

benlivengood wrote: post #15
LAC-Tech wrote: post #7

> The protocol presents a leader-less solution, where any node can become an opportunistic coordinator for an operation. Does leader = master here? My first reaction is that this is a multi-master system but I can't quite unpack "opportunistic coordinator".

For MultiPaxos the leader is sticky to avoid additional round-trips for each transaction. Basically until a configurable timeout all transactions are attempted at the last node to lead an accepted transaction, and only if that times out or fails is a new leader of a transaction proposed.

Again, leader = master here right? As in the node that can both read & write.

So this is different from multi-master in that one node is preferred until it's not, as opposed to any node being able to accept writes at any time.

Re: Scalable but Wasteful, or why fast replication protocols are slow

#20

Post by historyloop »

rdw wrote: post #8

The hypothesis in the article could be correct (that industry is not adopting new academic innovations because they fail in the real world). Based on my experience in this industry, though, it could just be that there isn't a super strong connection between academia and the people implementing these kinds of systems. I've had many conversations with my academically-minded friends where they're astonished that we have…

It leaves a bad taste in my mouth when an argument in that area starts using fake math to drive a point home. Top-tier engineers spend 6 months to have 20% improvement that could be achieved by spending 20% more on hardware?

Where IS all this coming from? Why is it acceptable to pull out a few random numbers out of our ass and conclude that it's always best to buy more hardware?

I know Google literally invented HTTP/2 and HTTP/3 to shave a few % off their hardware costs. And they had to not only implement this in Chrome and Google.com but to make it a world-wide standard. So surely they must be the idiots putting top-tier engineers for 6 months (or more) on such optimization efforts, instead of spending more on hardware? I don't know.

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