4 ms·

I really like the manycores approach, but we haven’t seen it come to fruition — at least not on general purpose machines. I think a machine that exposes each su

by sargun 1y ago

I really like the manycores approach, but we haven’t seen it come to fruition — at least not on general purpose machines. I think a machine that exposes each subset of cores as a NUMA node and doesn’t try to flatten memory across the entire set of cores might be a much more workable approach. Otherwise the interconnect becomes the scaling limit quickly (all cores being able to access all memory at speed). Erlang, at least the programming model, lends itself well to this, where each process has a local heap. If that can stay resident to a subsection of the CPU, that might lend itself better to a reasonably priced many core architecture.

zozbot234 1y ago
> Erlang, at least the programming model, lends itself well to this, where each process has a local heap. That loosely describes plenty of multithreaded workloads, perhaps even most of them. A thread that doesn't keep its memory writes "local" to itself as much as possible will run into heavy contention with other threads and performance will suffer a lot. It's usual to try and write multithreaded workloads in a way that tries to minimize the chance of contention, even though this may not involve a literal "one local heap per core".
- bglusman 1y ago
  Yes, but in Erlang, everything on every process is immutable and nothing is ever trying to write anywhere besides locally. Every variable assignment leaves the previous memory unchanged and fully accessible to anything directly referencing it.
felixgallo 1y ago
Paraphrasing the late great Joe Armstrong, the great thing about Erlang as opposed to just about any other language is that every year the same program gets twice as fast as last year. Manycores hasn't succeeded because frankly the programming model of essentially every other language is stuck in 1950. I, the program, am the entire and sole thing running on this computer, and must manually manage resources to match its capabilities. Hence async/await, mutable memory, race checkers, function coloring, all that nonsense. If half the effort spent straining to get the ghost PDP-11 ruling all the programming languages had been spent on cleaning up the (several) warts in the actor model and its few implementations, we'd all be driving Waymos on Jupiter by now.
- ncgl 1y ago
  Can you explain the joe armstrong quote a bit to someone not familiar with the language?
  - sam-cop-vimes 1y ago
    Erlang's runtime system, the BEAM, automatically takes care of scheduling the execution of lightweight erlang processes across many cpus/cores. So a well written Erlang program can be sped up almost linearly by adding more cpus/cores. And since we are seeing more and more cores being crammed into cpus each year, what Joe meant is that by deploying your code on the latest cpu, you've doubled the performance without touching your code.
- RossBencina 1y ago
  I'm curious, which actor model warts are you referring to exactly? [The obvious candidates from my point of view are (1) it's an abstract mathematical model with dispersed application/implementations, most of which introduce additional constraints (in other words, there is no central theory of the actor model implementation space), and (2) the message transport semantics are fixed: the model assumes eventual out-of-order delivery of an unbounded stream of messages. I think they should have enumerated the space of transport capabilities including ordered/unordered, reliable/unreliable within the core model. Treatment of bounded queuing in the core model would also be nice, but you can model that as an unreliable intermediate actor that drops messages or implements a backpressure handshake when the queue is full.]
  - felixgallo 1y ago
    I don't think either of those are particularly problematic. The actor model as implemented by Erlang is concrete and robust enough. The big problems with the actor model are, in my opinion, around (1) speed optimizations for immutable memory and message passing (currently, there's a great deal of copying and pointer chasing involved, which can be slow and is a ripe area for optimization), (2) (for Erlang) speed and QOL improvements for math and strings (Erlang historically is not about fast math or string handling, but both of those do comprise a great deal of general purpose programming), (3) (for Erlang) operational QOL misc improvements (e.g. existing distribution, ets, amnesia, failover, hot upgrade, node deployment, build process range from arcane (amnesia, hot upgrades, etc.all the way up to covered-in-terrifying-spiders (e.g. debugging queuing issues, rebar3))
toast0 1y ago
> think a machine that exposes each subset of cores as a NUMA node and doesn’t try to flatten memory across the entire set of cores might be a much more workable approach. Otherwise the interconnect becomes the scaling limit quickly (all cores being able to access all memory at speed). Epyc has a mode where it does 4 numa nodes per socket, IIRC. It seems like that should be good if your software is NUMA aware or NUMA friendly. But most of the desktop class hardware has all the cores sharing a single memory controller anyway, so if you had separate NUMA nodes, it wouldn't reflect reality. Reducing cross core communication (NUMA or not) is the key to getting high performance parallelism. Erlang helps because any cross process communication is explicit, so there's no hidden communication as can sometimes happen in languages with shared memory between threads. (Yes, ets is shared, but it's also explicit communication in my book)
leoc 1y ago
Who knows what will really happen, but there have been rumours of significant core-count bumps in Ryzen 6, which would edge the mainstream significantly closer to manycore.
to11mtm 1y ago
> Erlang, at least the programming model, lends itself well to this, where each process has a local heap. If that can stay resident to a subsection of the CPU, that might lend itself better to a reasonably priced many core architecture. I tend to agree. Where it gets -really- interesting to think about, are concepts like 'core parking' actors of a given type on specific cores; e.x. 'somebusinessprocess' actor code all happens on a specific fixed set of cores and 'account' actors run on a different fixed set of cores, versus having all the cores going back and forth between both. Could theoretically get a benefit due to instruction cache being very consistent per core, giving benefits due to the mechanical sympathy (I think Disruptors also take advantage of this). On the other hand, it may not be as big a benefit, in the sense that cross process writes are cross core writes and those tend to lead to their own issues... fun to think about.
- LtdJorge 1y ago
  The BEAM launches a scheduler process per CPU thread in SMP mode, although I don't know if it moves Erlang processes between them.
  - oxidant 1y ago
    The behavior is configurable and the default is unbound. https://www.erlang.org/doc/apps/erts/erl_cmd.html#%2Bsbt https://www.erlang.org/doc/apps/erts/erl_cmd.html#%2Bsbt

by sargun 1y ago

zozbot234 1y ago

> Erlang, at least the programming model, lends itself well to this, where each process has a local heap. That loosely describes plenty of multithreaded workloads, perhaps even most of them. A thread that doesn't keep its memory writes "local" to itself as much as possible will run into heavy contention with other threads and performance will suffer a lot. It's usual to try and write multithreaded workloads in a way that tries to minimize the chance of contention, even though this may not involve a literal "one local heap per core".

bglusman 1y ago
Yes, but in Erlang, everything on every process is immutable and nothing is ever trying to write anywhere besides locally. Every variable assignment leaves the previous memory unchanged and fully accessible to anything directly referencing it.

felixgallo 1y ago

Paraphrasing the late great Joe Armstrong, the great thing about Erlang as opposed to just about any other language is that every year the same program gets twice as fast as last year. Manycores hasn't succeeded because frankly the programming model of essentially every other language is stuck in 1950. I, the program, am the entire and sole thing running on this computer, and must manually manage resources to match its capabilities. Hence async/await, mutable memory, race checkers, function coloring, all that nonsense. If half the effort spent straining to get the ghost PDP-11 ruling all the programming languages had been spent on cleaning up the (several) warts in the actor model and its few implementations, we'd all be driving Waymos on Jupiter by now.

ncgl 1y ago
Can you explain the joe armstrong quote a bit to someone not familiar with the language?
- sam-cop-vimes 1y ago
  Erlang's runtime system, the BEAM, automatically takes care of scheduling the execution of lightweight erlang processes across many cpus/cores. So a well written Erlang program can be sped up almost linearly by adding more cpus/cores. And since we are seeing more and more cores being crammed into cpus each year, what Joe meant is that by deploying your code on the latest cpu, you've doubled the performance without touching your code.
RossBencina 1y ago
I'm curious, which actor model warts are you referring to exactly? [The obvious candidates from my point of view are (1) it's an abstract mathematical model with dispersed application/implementations, most of which introduce additional constraints (in other words, there is no central theory of the actor model implementation space), and (2) the message transport semantics are fixed: the model assumes eventual out-of-order delivery of an unbounded stream of messages. I think they should have enumerated the space of transport capabilities including ordered/unordered, reliable/unreliable within the core model. Treatment of bounded queuing in the core model would also be nice, but you can model that as an unreliable intermediate actor that drops messages or implements a backpressure handshake when the queue is full.]
- felixgallo 1y ago
  I don't think either of those are particularly problematic. The actor model as implemented by Erlang is concrete and robust enough. The big problems with the actor model are, in my opinion, around (1) speed optimizations for immutable memory and message passing (currently, there's a great deal of copying and pointer chasing involved, which can be slow and is a ripe area for optimization), (2) (for Erlang) speed and QOL improvements for math and strings (Erlang historically is not about fast math or string handling, but both of those do comprise a great deal of general purpose programming), (3) (for Erlang) operational QOL misc improvements (e.g. existing distribution, ets, amnesia, failover, hot upgrade, node deployment, build process range from arcane (amnesia, hot upgrades, etc.all the way up to covered-in-terrifying-spiders (e.g. debugging queuing issues, rebar3))

toast0 1y ago

> think a machine that exposes each subset of cores as a NUMA node and doesn’t try to flatten memory across the entire set of cores might be a much more workable approach. Otherwise the interconnect becomes the scaling limit quickly (all cores being able to access all memory at speed). Epyc has a mode where it does 4 numa nodes per socket, IIRC. It seems like that should be good if your software is NUMA aware or NUMA friendly. But most of the desktop class hardware has all the cores sharing a single memory controller anyway, so if you had separate NUMA nodes, it wouldn't reflect reality. Reducing cross core communication (NUMA or not) is the key to getting high performance parallelism. Erlang helps because any cross process communication is explicit, so there's no hidden communication as can sometimes happen in languages with shared memory between threads. (Yes, ets is shared, but it's also explicit communication in my book)

leoc 1y ago

Who knows what will really happen, but there have been rumours of significant core-count bumps in Ryzen 6, which would edge the mainstream significantly closer to manycore.

to11mtm 1y ago

> Erlang, at least the programming model, lends itself well to this, where each process has a local heap. If that can stay resident to a subsection of the CPU, that might lend itself better to a reasonably priced many core architecture. I tend to agree. Where it gets -really- interesting to think about, are concepts like 'core parking' actors of a given type on specific cores; e.x. 'somebusinessprocess' actor code all happens on a specific fixed set of cores and 'account' actors run on a different fixed set of cores, versus having all the cores going back and forth between both. Could theoretically get a benefit due to instruction cache being very consistent per core, giving benefits due to the mechanical sympathy (I think Disruptors also take advantage of this). On the other hand, it may not be as big a benefit, in the sense that cross process writes are cross core writes and those tend to lead to their own issues... fun to think about.

LtdJorge 1y ago
The BEAM launches a scheduler process per CPU thread in SMP mode, although I don't know if it moves Erlang processes between them.
- oxidant 1y ago
  The behavior is configurable and the default is unbound. https://www.erlang.org/doc/apps/erts/erl_cmd.html#%2Bsbt https://www.erlang.org/doc/apps/erts/erl_cmd.html#%2Bsbt