Peter Maydell <peter.mayd...@linaro.org> writes: > On 10 July 2017 at 15:28, Alex Bennée <alex.ben...@linaro.org> wrote: >> While the SoftMMU is not emulating the target MMU of a system there is >> a relationship between its page size and that of the target. If the >> target MMU is full featured the functions called to re-fill the >> entries in the SoftMMU entries start moving up the perf profiles. If >> we can we should try and prevent too much thrashing around by having >> the page sizes the same. >> >> Ideally we should use TARGET_PAGE_BITS_MIN but that potentially >> involves a fair bit of #include re-jigging so I went for 10 bits (1k >> pages) which I think is the smallest of all our emulated systems. > > The figures certainly show an improvement, but it's not clear > to me why this is related to the target's page size rather than > just being a "bigger is better" kind of thing?
Well this was driven by a discussion with Pranith last week. In his (admittedly memory intensive) bench-marking he was seeing around 30% overhead is coming from mmu related functions with the hottest being get_phys_addr_lpae() followed by address_space_do_translate(). We theorised that even given the high hit rate of the fast path the slow path was triggered by moving over SoftMMU's effective page boundary. A quick experiment in extending the size of the TLB made his hot spots disappear. I don't see quite such a hot-spot in my simple boot/build benchmark test but after helper_lookup_tb_ptr quite a lot of hits are part of the re-fill chain: 16.37% qemu-system-aar qemu-system-aarch64 [.] helper_lookup_tb_ptr 3.43% qemu-system-aar qemu-system-aarch64 [.] victim_tlb_hit 2.73% qemu-system-aar qemu-system-aarch64 [.] tlb_set_page_with_attrs 2.60% qemu-system-aar qemu-system-aarch64 [.] get_phys_addr_lpae 2.36% qemu-system-aar qemu-system-aarch64 [.] qht_lookup 1.53% qemu-system-aar qemu-system-aarch64 [.] arm_regime_tbi1 1.37% qemu-system-aar qemu-system-aarch64 [.] tcg_optimize 1.34% qemu-system-aar qemu-system-aarch64 [.] tcg_gen_code 1.31% qemu-system-aar qemu-system-aarch64 [.] arm_regime_tbi0 1.28% qemu-system-aar qemu-system-aarch64 [.] address_space_ldq_le 1.22% qemu-system-aar qemu-system-aarch64 [.] object_dynamic_cast_assert 1.11% qemu-system-aar qemu-system-aarch64 [.] address_space_translate_internal 1.03% qemu-system-aar qemu-system-aarch64 [.] tb_htable_lookup 0.98% qemu-system-aar qemu-system-aarch64 [.] get_page_addr_code 0.98% qemu-system-aar qemu-system-aarch64 [.] address_space_do_translate 0.87% qemu-system-aar qemu-system-aarch64 [.] object_class_dynamic_cast_assert 0.82% qemu-system-aar qemu-system-aarch64 [.] get_phys_addr 0.75% qemu-system-aar qemu-system-aarch64 [.] tb_cmp 0.63% qemu-system-aar qemu-system-aarch64 [.] liveness_pass_1 0.59% qemu-system-aar qemu-system-aarch64 [.] helper_le_ldq_mmu -- Alex Bennée
