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runtime: support a two-level arena map
Currently, the heap arena map is a single, large array that covers every possible arena frame in the entire address space. This is practical up to about 48 bits of address space with 64 MB arenas. However, there are two problems with this: 1. mips64, ppc64, and s390x support full 64-bit address spaces (though on Linux only s390x has kernel support for 64-bit address spaces). On these platforms, it would be good to support these larger address spaces. 2. On Windows, processes are charged for untouched memory, so for processes with small heaps, the mostly-untouched 32 MB arena map plus a 64 MB arena are significant overhead. Hence, it would be good to reduce both the arena map size and the arena size, but with a single-level arena, these are inversely proportional. This CL adds support for a two-level arena map. Arena frame numbers are now divided into arenaL1Bits of L1 index and arenaL2Bits of L2 index. At the moment, arenaL1Bits is always 0, so we effectively have a single level map. We do a few things so that this has no cost beyond the current single-level map: 1. We embed the L2 array directly in mheap, so if there's a single entry in the L2 array, the representation is identical to the current representation and there's no extra level of indirection. 2. Hot code that accesses the arena map is structured so that it optimizes to nearly the same machine code as it does currently. 3. We make some small tweaks to hot code paths and to the inliner itself to keep some important functions inlined despite their now-larger ASTs. In particular, this is necessary for heapBitsForAddr and heapBits.next. Possibly as a result of some of the tweaks, this actually slightly improves the performance of the x/benchmarks garbage benchmark: name old time/op new time/op delta Garbage/benchmem-MB=64-12 2.28ms ± 1% 2.26ms ± 1% -1.07% (p=0.000 n=17+19) (https://perf.golang.org/search?q=upload:20180223.2) For #23900. Change-Id: If5164e0961754f97eb9eca58f837f36d759505ff Reviewed-on: https://go-review.googlesource.com/96779 Run-TryBot: Austin Clements <firstname.lastname@example.org> TryBot-Result: Gobot Gobot <email@example.com> Reviewed-by: Rick Hudson <firstname.lastname@example.org>
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