Review: Dark Silicon and the End of Multicore Scaling

Paper link: p365-esmaeilzadeh

Premise

Single core scaling stopped some number of years ago. But we’re okay, because we now have the “multi-core revolution” (I realllly hate that phrase). Or are we?

It turns out that another, very real wall is blocking future progress — power density. Already CPU designs strive mightily to shutdown or slowdown underutilized circuitry to save power. But we’re reaching the point where processors simply won’t ever be able to run “full-out”.

This paper explores the future of scaling by extrapolating based on a wide range of processors over time, and they find, rather compellingly, that we’re pretty much boned.

Details

The paper is pretty dense, but it boils down to the creation of a model parameterized by processor features:

  • number of cores
  • number of threads
  • CPU frequency
  • L1/L2/DRAM cache size, speed, miss rate
  • DRAM fetch width

and code features:

  • % code that is parallel
  • % read instructions

You can fit this model based on existing processors and their benchmark performance, as well as their power consumption and die area.

Once the model is created, now the fun starts — we can see how performance will scale with future process improvements (moving to 32, 22, 16…nm), or if we assume highly parallel code, we can see how to optimize our CPU design (lots of cores)!

The short of it

If we make optimistic assumptions for how power consumption drops with process scaling, we still end up with the sobering conclusion that we’re rapidly reaching the point where we won’t be able to power all of the parts of a chip. 128 cores aren’t as useful if we can’t keep them all running.

On the plus side, I can buy 1000 watt power supplies now, so I’m looking forward to my 16 processor, 16 core machine of the future. Welcome to the multiprocessor revolution!

Review: DThreads: Efficient Deterministic Multithreading

I’ve decided to try and be more pro-active about reading papers from the community. Since I tend to forget everything I read, it’s best I write down somewhere my thoughts for later reference — I’ve decided to post these reviews here in case they prove useful to others.

PDF of the paper

Overview:

This paper follows one of the latest fads in systems (NB: academics have fads just like fashionistas) — deterministic threading. The main idea behind all of these papers is to try and allow users to make use of multi-core/multi-threaded programming, but at the same time have it be deterministic. This is helpful as it ensures the behavior we see when testing is the same as what we see at runtime, and it also helps make it easier to recreate test failures.

This is a non-trivial problem, and it has a lot of work behind it already. The contribution with dthreads is that it, (a) claims high performance, and (b) provides a drop-in replacement for existing threads.

How does it work:

There’s a standard trick for d-threading, and DThreads uses it as well. It’s somewhat a kin to transactions in databases. First, split each thread into a separate world that can’t communicate with the other threads. Buffer all of the updates that would go to shared memory. Then, at a synchronization point (mutex lock, condition var, etc.), wait for all of the threads to finish their work, and then apply the shared memory changes in a deterministic fashion.

The DThreads way of handling this is to run each thread as a separate process. Whenever a commit point is reached, the processes determine which parts of memory they’ve changed, and commit them to shared memory at that time.

There are a number of things that need to be handled to make this all possible — you need to provide a deterministic malloc, you have to ensure threads are created and destroyed in a deterministic way, etc.

Why aren’t we using it today?

Two reasons, really.

Speed

DThreads is faster then previous work on the subject, but still can end up a lot slower then regular threading. This isn’t unexpected — you have to do more work, so you end up paying for it.

Correctness

DThreads, while being a drop-in replacement for pthreads, isn’t really a drop-in replacement for pthreads. Unfortunately for deterministic threading proponents, this type of code:

Thread 1:

while not done:
  sleep(0)

Thread 2:
... do some work ...
done = 1

where synchronization is handled outside the scope of the threading library is all too common in the wild. This forces the use of more expensive techniques then DThreads, which keeps determistic threading, for the moment, an academic exercise.

What did I think of the paper

It’s well written (which I expect of all SOSP papers), and presents a bunch of interesting tricks for improving performance and making things work. The fact that it requires all synchronization be performed through pthreads calls is on the face of it, reasonable, but sadly, it is far from realistic when it comes to running everyday applications.

Still, fun to read.