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ae3e22a689
For some workloads such as arm bootup, tb_phys_hash is performance-critical. The is due to the high frequency of accesses to the hash table, originated by (frequent) TLB flushes that wipe out the cpu-private tb_jmp_cache's. More info: https://lists.nongnu.org/archive/html/qemu-devel/2016-03/msg05098.html To dig further into this I modified an arm image booting debian jessie to immediately shut down after boot. Analysis revealed that quite a bit of time is unnecessarily spent in tb_phys_hash: the cause is poor hashing that results in very uneven loading of chains in the hash table's buckets; the longest observed chain had ~550 elements. The appended addresses this with two changes: 1) Use xxhash as the hash table's hash function. xxhash is a fast, high-quality hashing function. 2) Feed the hashing function with not just tb_phys, but also pc and flags. This improves performance over using just tb_phys for hashing, since that resulted in some hash buckets having many TB's, while others getting very few; with these changes, the longest observed chain on a single hash bucket is brought down from ~550 to ~40. Tests show that the other element checked for in tb_find_physical, cs_base, is always a match when tb_phys+pc+flags are a match, so hashing cs_base is wasteful. It could be that this is an ARM-only thing, though. UPDATE: On Tue, Apr 05, 2016 at 08:41:43 -0700, Richard Henderson wrote: > The cs_base field is only used by i386 (in 16-bit modes), and sparc (for a TB > consisting of only a delay slot). > It may well still turn out to be reasonable to ignore cs_base for hashing. BTW, after this change the hash table should not be called "tb_hash_phys" anymore; this is addressed later in this series. This change gives consistent bootup time improvements. I tested two host machines: - Intel Xeon E5-2690: 11.6% less time - Intel i7-4790K: 19.2% less time Increasing the number of hash buckets yields further improvements. However, using a larger, fixed number of buckets can degrade performance for other workloads that do not translate as many blocks (600K+ for debian-jessie arm bootup). This is dealt with later in this series. Backports commit 42bd32287f3a18d823f2258b813824a39ed7c6d9 from qemu |
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| crypto | ||
| default-configs | ||
| docs | ||
| fpu | ||
| hw | ||
| include | ||
| qapi | ||
| qobject | ||
| qom | ||
| scripts | ||
| target-arm | ||
| target-i386 | ||
| target-m68k | ||
| target-mips | ||
| target-sparc | ||
| tcg | ||
| util | ||
| aarch64.h | ||
| aarch64eb.h | ||
| accel.c | ||
| arm.h | ||
| armeb.h | ||
| CODING_STYLE | ||
| configure | ||
| COPYING | ||
| COPYING.LIB | ||
| cpu-exec-common.c | ||
| cpu-exec.c | ||
| cpus.c | ||
| cputlb.c | ||
| exec.c | ||
| gen_all_header.sh | ||
| glib_compat.c | ||
| HACKING | ||
| header_gen.py | ||
| ioport.c | ||
| LICENSE | ||
| m68k.h | ||
| Makefile | ||
| Makefile.objs | ||
| Makefile.target | ||
| memory.c | ||
| memory_mapping.c | ||
| mips.h | ||
| mips64.h | ||
| mips64el.h | ||
| mipsel.h | ||
| powerpc.h | ||
| qapi-schema.json | ||
| qemu-log.c | ||
| qemu-timer.c | ||
| rules.mak | ||
| softmmu_template.h | ||
| sparc.h | ||
| sparc64.h | ||
| tcg-runtime.c | ||
| translate-all.c | ||
| translate-all.h | ||
| translate-common.c | ||
| unicorn_common.h | ||
| VERSION | ||
| vl.c | ||
| vl.h | ||
| x86_64.h | ||