runtime: add per-G shadows of writeBarrier.enabled

[sorear]

1. A large fraction of static instructions are used to implement the write barrier enabled check, which currently always uses an absolute memory reference.

2. On supported RISC architectures accessing data at a small offset from a pointer takes fewer instructions than accessing data at an absolute offset. On all(?) supported architectures, it takes fewer bytes.

3. Reserving a register to point at the runtime.writeBarrier struct is possible, but would be a difficult tradeoff. However, many architectures already reserve a register to point at the executing G. If we could check write barrier status relative to G, it would save instructions on all RISC architectures.

4. There are many Gs. Since the enabled flag is updated very rarely, it would be possible to update some or all Gs whenever the master flag is updated. There is a tradeoff of which Gs to update: all of them, those that have Ms, or those that have Ps.

5. I have a proof of concept patch against the riscv tree (https://review.gerrithub.io/#/c/357282/ or sorear/riscv-go@dab0f89). I can rebase it against master if there is interest. This patch takes the approach of keeping Gs updated if they are referenced by any M; thus the additional STW latency scales as the number of Ms, but there is less potential for a race with exitsyscall.

6. It is far from clear that I have accounted for all possible races, especially with regards to asynchronous cgo callbacks that could(?) create new Ms at any time.

7. Initial results measured by .text size on cmd/compile:

            before   after    %
   386      5730855  5731751 +0.016
   amd64    6764675  6765155 +0.007
   arm      6155060  6081080 -1.202
   arm64    5850320  5725184 -2.139
   mips64   7297336  7173880 -1.692
   mips     7159648  7097940 -0.862
   ppc64    6120800  6058392 -1.020
   riscv    3986656  3924656 -1.555
   s390x    8253200  8343808 +1.098
   

   ppc64 and s390x do not benefit from this patch alone as the current backends for those architectures are unable to use the G register as a base register for memory accesses. For ppc64 I did the measurement with a one-line change that enables G as a base register, for s390x I tried to make a similar change but was not able to get it to work.

   It may make sense to exclude s390x from the code generation change since s390x can fetch from an absolute address in one instruction; currently the code generation is conditionalized exclusively on hasGReg.

   The demonstration patch keeps the per-G shadows updated even on 386 and amd64 where they are not used. There could be conditionals added to the runtime to avoid that overhead.

8. I do not have any physical user-programmable hardware of the most affected architectures. While a 2.1% reduction in static instructions for arm64 looks nice on paper, it's moot if it turns out to make things slower for whatever reason.

9. Is this strategically desirable? It makes moving away from STW at GC phase changes marginally more difficult, increases g size, and might cause other problems I'm not considering.

cc @josharian @aclements @randall77

[josharian]

Loosely related: #15245 and #19838.

cc @cherrymui

[rsc]

Personally, I would much rather have lower complexity in the GC code than save 1% in text size. It sounds like the proposed changes are quite subtle and difficult to maintain, and that's already true of many other parts of the GC. I'm not convinced this is the right tradeoff, at least not right now.

[rsc]

-> @aclements for decision

[sorear]

Shall I make a patch against current master so someone can run benchmarks on affected architectures?

[aclements]

I'm somewhat concerned about the potential complexity of this, though your current implementation seems pretty simple.

This has interesting interactions with some changes I have on my list to experiment with. Primarily, I want to try an SSB-style write barrier where the code to append to a local write barrier buffer is inlined and only the overflow involves a call. This would need efficient access to a per-P buffer (which could be mirrored into the G or wherever's best), which probably needs to be hooked in at exactly the same places as your change.

There are many Gs. Since the enabled flag is updated very rarely, it would be possible to update some or all Gs whenever the master flag is updated. There is a tradeoff of which Gs to update: all of them, those that have Ms, or those that have Ps.

I don't think your CL's current approach of iterating over the Ms to set the flag during STW is the right approach. There can be a huge number of Ms, especially in syscall- or cgo-heavy code, so this could significantly increase STW time. It would be much better to do this over the Ps. Doing it over Ps also has a nice parallel with the write barriers themselves: code isn't allowed to execute a write barrier unless it has a P. There's probably some subtlety here, but I think the right places to hook this in would be acquirep1 and execute. I don't think you'd have to do anything specifically during STW since starting the world requires re-acquiring all of the Ps.

A large fraction of static instructions are used to implement the write barrier enabled check, which currently always uses an absolute memory reference.

Interesting. Do you have a number for this?

I do not have any physical user-programmable hardware of the most affected architectures. While a 2.1% reduction in static instructions for arm64 looks nice on paper, it's moot if it turns out to make things slower for whatever reason.

Do you have performance numbers for the architectures you do have access to? It would be particularly interesting to know the results for golang.org/x/benchmarks/garbage, which spends roughly 25% of the time with write barriers enabled.

[rsc]

Any progress on making a decision here?

[aclements]

@sorear, in principle I think we're okay with this, but we'd like to see a CL with the changes I mentioned above and, if possible, performance numbers.

I'm somewhat concerned about updating the shadow on the g0s. I definitely don't want to loop over the Ms because of the impact on STW time. However, code running on an M that isn't associated with a P isn't allowed to have write barriers anyway, so in principle updating the shadow for these g0s only needs to be done in acquirep.

[aclements]

@cherrymui points out that, rather than storing the value of the write barrier flag in the G, we could store a pointer to it. That would be far simpler and more robust, but would probably still reduce the inlined check sequence (though not as much).

[sorear]

That'd help on mips and possibly ppc64, but for arm, aarch64, and riscv64 global variable access is already 2-instructions.

(My copy of Go is generating a 3-instruction sequence for global variable access ADRP-ADD-LDR, but clang and gcc generate ADRP-LDR with the immediate folded into the LDR and there's no reason in principle the Go aarch64 backend couldn't be taught the same)

I'll make some time later to make an up to date CL and benchmark it on ARM hardware.

[rsc]

Marking this accepted, though of course if the performance is no good then it doesn't have to go in.