Re: [Qemu-devel] [RFC v1 01/11] tcg: move tb_find_fast outside the tb_lock critical section

["Emilio G. Cota" <cota@braap.org>]

On Mon, Mar 21, 2016 at 22:08:06 +0000, Peter Maydell wrote:
> It is not _necessary_, but it is a performance optimization to
> speed up the "missed in the TLB" case. (A TLB flush will wipe
> the tb_jmp_cache table.) From the thread where the move-to-front-of-list
> behaviour was added in 2010, benefits cited:

(snip)
> I think what's happening here is that for guest CPUs where TLB
> invalidation happens fairly frequently (notably ARM, because
> we don't model ASIDs in the QEMU TLB and thus have to flush
> the TLB on any context switch) the case of "we didn't hit in
> the TLB but we do have this TB and it was used really recently"
> happens often enough to make it worthwhile for the
> tb_find_physical() code to keep its hash buckets in LRU order.
> 
> Obviously that's all five year old data now, so a pinch of
> salt may be indicated, but I'd rather we didn't just remove
> the optimisation without some benchmarking to check that it's
> not significant. A 2x difference is huge.

Good point. Most of my tests have been on x86-on-x86, and the
difference there (for many CPU-intensive benchmarks such as SPEC) was
negligible.

Just tested the current master booting Alex' debian ARM image, without
LRU, and I see a 20% increase in boot time.

I'll add per-bucket locks to keep the same behaviour without hurting
scalability.

Thanks,

		Emilio