Use a higher tier-up threshold for JIT code

[brandtbucher]

Our current tier-up threshold is 16, which was chosen a while ago because:

• in theory, it gives some of our 16-bit branch counters time to stabilize
• it seemed to work fine in practice

It turns out that we're leaving significant performance and memory improvements on the table by not using higher thresholds. Here are the results of some experiments I ran:

warmup	speedup	memory	traces created	traces executed	uops executed
64	+0.3%	-1.2%	-8.0%	-0.1%	+0.2%
256	+1.0%	-2.6%	-22.0%	-0.7%	-1.3%
1024	+1.2%	-3.2%	-38.6%	-3.0%	-1.5%
2048	+1.1%	-3.3%	-44.9%	-12.4%	-3.8%
4096	+2.1%	-3.6%	-52.2%	-11.2%	-3.1%
8192*	+2.0%	-3.4%	-59.2%	-12.8%	-3.1%
16384*	+2.0%	-3.6%	-65.2%	-14.5%	-4.7%
32768*	+1.8%	-3.8%	-73.1%	-18.3%	-7.1%
65536*	+1.4%	-3.9%	-79.7%	-21.9%	-9.2%

* For warmups above 4096, exponential backoff is disabled.

Based on these numbers, I think 4096 as a new threshold makes sense (2% faster and 3% less memory without significant hits to the amount of work we actually do in JIT code). I'll open a PR.

My next steps will be conducting similar experiments with higher side-exit warmup values, and then lastly with different JIT_CLEANUP_THRESHOLD values.

Linked PRs

• GH-126795: Increase the JIT threshold from 16 to 4096 #126816
• GH-126795: Increase the JIT side-exit threshold from 64 to 4096 #127155

[terryjreedy]

Is 'speedup' comparing to result for 16?

I have no idea how likely significant correlations between parameters are for this problem, but in general, when doing multidimensional optimization 1 dimension at a time, I would recheck after doing all dimensions that earlier settings are still optimal.

[brandtbucher]

Is 'speedup' comparing to result for 16?

Yes! Sorry if that wasn't clear.

I have no idea how likely significant correlations between parameters are for this problem, but in general, when doing multidimensional optimization 1 dimension at a time, I would recheck after doing all dimensions that earlier settings are still optimal.

Yeah, that's a good idea. I don't know if I'll do another full sweep, but spot-checking the "neighbors" of the current value over time seems useful.

[alonme]

@brandtbucher I assume that powers of 2 are used because of the exponential backoff?
If so, and given that disabling the exponential backoff doesn't seem to have a large negative effect i think that it might be worth searching the space between 2048 and 8192 for the maximum point?

[brandtbucher]

I assume that powers of 2 are used because of the exponential backoff?

It's nice, but they aren't needed (our exponential backoff works fine with non-power-of-two initial values). I mainly chose powers of two because it's a pretty efficient way to search a half-open range of possible values. ;)

If so, and given that disabling the exponential backoff doesn't seem to have a large negative effect i think that it might be worth searching the space between 2048 and 8192 for the maximum point?

I'd like to avoid overfitting to the benchmarks. There's not going to be some "best" number, just a range of values that work well in practice. Being in the right order of magnitude is probably good enough, especially since good warmup values are very sensitive to different workloads and platforms (and as Terry mentioned, we'll probably want to continue tweaking the values over time).

[brandtbucher]

(Plus each benchmarking run takes several hours, and there's a clear plateau near the current chosen value.)

[brandtbucher]

The results of similar experiments with the threshold for warming up side-exits (currently set at 64):

warmup	speedup	memory	traces created	traces executed	uops executed
256	-0.3%	-0.1%	-26.6%	-0.3%	-0.1%
1024	+0.1%	-0.4%	-50.6%	-2.2%	-1.7%
2048	-0.0%	-0.5%	-58.9%	-1.7%	-0.6%
4096	+0.4%	-0.5%	-63.6%	+3.2%	-0.3%
8192*	+0.2%	-0.6%	-70.6%	-2.0%	-2.0%
16384*	+0.1%	-0.7%	-75.3%	-4.9%	-3.6%
65536*	-0.0%	-0.8%	-80.3%	-12.2%	-7.1%

The results are less dramatic here, but it does seem like switching to 4096 here too would result in small performance improvements and memory savings, with no real hit to uops executed.

Note that these new measurements were taken after the other threshold change to 4096 landed, so they accurately depict the improvements we'd see with the new values.