Feature #14038: Use rb_execution_context_t instead of rb_thread_t to represent execution context

• Status: Closed
• Assignee: ko1 (Koichi Sasada)
• Target version: 2.5

[ko1 (Koichi Sasada)]

Summary

This ticket proposes the following three changes.

• (1) Separate rb_execution_context_t from rb_thread_t.
• (2) Allocate rb_fiber_t for each Thread even if Threads don't make Fiber.
• (3) Use rb_execution_context_t *ec to represent VM states instead of rb_thread_t *th

Current patch is here:

https://github.com/ruby/ruby/compare/trunk...ko1:sep_con
https://github.com/ruby/ruby/compare/trunk...ko1:sep_con.patch

Background

Recently Eric Wong introduced rb_execution_context_t to represent the "context", state of VM (program counter, stack pointer and so on).

You can find this discussion around this mailing list threads:
[ ruby-core:81054] Re: [ruby-cvs:65407] normal:r58236 (trunk): thread.c: comments on M:N threading [ci skip]

Before introduction of rb_execution_context_t, VM status are contained by threads (rb_thread_t) and we need to copy status from rb_thread_t to switch Fiber context. I gathered all of states into rb_execution_context_t and now we only need to copy one sequential data rb_execution_context_t to switch Fiber context.

Proposal

Proposal (1) Separate rb_execution_context_t from rb_thread_t.

On this ticket I propose to separate rb_execution_context_t from rb_thread_t and rb_thread_t only contains pointer to separated rb_execution_context_t data. rb_execution_context_t memory are managed by rb_fiber_t.

Before:

struct rb_thread_t {
  ...;
  rb_execution_context_t ec;
  ...;
}

After:

struct rb_thread_t {
  ...;
  rb_execution_context_t *ec; /* points rb_fiber_t::ec_body */
  ...;
}

With this change, we only need to swap ec pointers to switch Fiber context.

Proposal (2) Allocate rb_fiber_t for each Thread even if Threads don't make Fiber.

Now root Fiber (and rb_fiber_t) is allocated just after another Fiber is created.
For proposal (1), we need to allocate rb_fiber_t to prepare ec body.

We can optimize this allocation with several techniques, but it introduce complexity. So that I allocated rb_fiber_t for each thread.

Proposal (3) Use rb_execution_context_t *ec to represent VM states instead of rb_thread_t *th

Now many functions accept rb_thread_t *th as first argument to represent VM states. For now most of states are in rb_execution_context_t so that we need to introduce indirect access th->ec->... to access VM states.

To overcome this performance overhead, we need to use ec instead of th as the first argument.

Evaluation

Comparison with trunk and modified version, vm2_fiber_switch benchmarks 10% speedup (not so big impact).

name                    trunk   modified
loop_whileloop2         0.113      0.116
vm2_fiber_switch*       2.500      2.242

Speedup ratio: compare with the result of `trunk' (greater is better)
name                    modified
loop_whileloop2            0.974
vm2_fiber_switch*          1.115

However, other micro-benchmarks show slowdown becuase of indirect access I explained at Proposal (3).

name              trunk   modified
loop_whileloop    0.436      0.444
vm1_simplereturn* 0.487      0.672

Speedup ratio: compare with the result of `trunk' (greater is better)
name    modified
loop_whileloop      0.981
vm1_simplereturn*   0.726

I believe we can improve by "Proposal (3)".

Foresight

For Guild proposal, context passing by ec is important.
I want to introduce many other APIs to accept ec as first argument and we don't need to call GET_THREAD() or GET_EC(), which cause system native thread-local-storage.

I will introduce this patch soon because this patch is only internal changes. Ruby users can't observe any incompatible changes (hopefully).

Thanks,
Koichi

[ko1 (Koichi Sasada)]

• Status changed from Open to Closed

Applied in changeset trunk|r60440.

---

Use rb_execution_context_t instead of rb_thread_t
to represent execution context [Feature #14038]

• vm_core.h (rb_thread_t): rb_thread_t::ec is now a pointer.
  There are many code using th to represent execution context
  (such as cfp, VM stack and so on). To access ec, they need to
  use th->ec->... (adding one indirection) so that we need to
  replace them by passing ec instead of th.

• vm_core.h (GET_EC()): introduced to access current ec. Also
  remove ruby_current_thread global variable.

• cont.c (rb_context_t): introduce rb_context_t::thread_ptr instead of
  rb_context_t::thread_value.

• cont.c (ec_set_vm_stack): added to update vm_stack explicitly.

• cont.c (ec_switch): added to switch ec explicitly.

• cont.c (rb_fiber_close): added to terminate fibers explicitly.

[aselder (Andrew Selder)]

FWIW, this changed seems to cause SegFaults on a regular basis on OS X

https://bugs.ruby-lang.org/issues/14714
https://bugs.ruby-lang.org/issues/14561
https://bugs.ruby-lang.org/issues/15308
https://bugs.ruby-lang.org/issues/14334