unicorn/qemu/include/exec/tb-hash.h

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/*
* internal execution defines for qemu
*
* Copyright (c) 2003 Fabrice Bellard
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2 of the License, or (at your option) any later version.
*
* This library is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this library; if not, see <http://www.gnu.org/licenses/>.
*/
#ifndef EXEC_TB_HASH_H
#define EXEC_TB_HASH_H
tb hash: hash phys_pc, pc, and flags with xxhash 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
2018-02-24 22:45:39 +00:00
#include "exec/exec-all.h"
#include "qemu/xxhash.h"
tb hash: hash phys_pc, pc, and flags with xxhash 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
2018-02-24 22:45:39 +00:00
tb-hash: improve tb_jmp_cache hash function in user mode Optimizations to cross-page chaining and indirect branches make performance more sensitive to the hit rate of tb_jmp_cache. The constraint of reserving some bits for the page number lowers the achievable quality of the hashing function. However, user-mode does not have this requirement. Thus, with this change we use for user-mode a hashing function that is both faster and of better quality than the previous one. Measurements: Note: baseline (i.e. speedup == 1x) is QEMU v2.9.0. - SPECint06 (test set), x86_64-linux-user. Host: Intel i7-6700K @ 4.00GHz 2.2x +-+--------------------------------------------------------------------------------------------------------------+-+ | | | jr | 2x +jr+multhash +....................................................+++++...................................+-+ | jr+hash |$$$ | | |$+$ | | ### $ | 1.8x +-+......................................................................#|#.$...................................+-+ | ++#+# $ | | |# # $ | 1.6x +-+....................................................................***.#.$....................++$$$..........+-+ | $$$ *+* # $ |$+$ | | ++$$$ ### $ * * # $ +++|$ $ | | ++###+$ # # $ * * # $ ### ****## $ | 1.4x +-+...................***+#.$.........***.#.$..........................*.*.#.$...........#+#$$.*++*|#.$..........+-+ | *+* # $ * * # $ * * # $ # # $ * *+# $ | | * * # $ +++++ * * # $ * * # $ *** # $ * * # $ ###$$ | 1.2x +-+...................*.*.#.$.***##$$.*.*.#.$..........................*.*.#.$.........*.*.#.$.*..*.#.$.***+#+$..+-+ | * * # $ *+* # $ * * # $ +++ * * # $ ++###$$ * * # $ * * # $ * * # $ | | ***##$$ * * # $ * * # $ * * # $ ***##$$ ++### * * # $ *** #+$ * * # $ * * # $ * * # $ | | *+*+#+$ ***##$$$ * * # $ * * # $ * * # $ *+* # $ ++####$$ ***+# * * # $ * * # $ * * # $ * * # $ * * # $ | 1x +-++-*+*+#+$+*+*+#-+$+*+*-#+$+*+*+#+$+*+*+#+$+*-*+#+$+***++#+$+*+*+#$$+*+*+#+$+*+*+#+$+*+*-#+$+*+-*+#+$+*+*+#+$-++-+ | * * # $ * * # $ * * # $ * * # $ * * # $ * * # $ * * # $ * * # $ * * # $ * * # $ * * # $ * * # $ * * # $ | | * * # $ * * # $ * * # $ * * # $ * * # $ * * # $ * * # $ * * # $ * * # $ * * # $ * * # $ * * # $ * * # $ | 0.8x +-+--***##$$-***##$$$-***##$$-***##$$-***##$$-***##$$-***###$$-***##$$-***##$$-***##$$-***##$$-****##$$-***##$$--+-+ astar bzip2 gcc gobmk h264ref hmmlibquantum mcf omnetpperlbench sjengxalancbmk hmean png: http://imgur.com/4UXTrEc Here I also tried the hash function suggested by Paolo ("multhash"): return ((uint64_t) (pc * 2654435761) >> 32) & (TB_JMP_CACHE_SIZE - 1); As you can see it is just as good as the other new function ("hash"), which is what I ended up going with. - SPECint06 (train set), x86_64-linux-user. Host: Intel i7-6700K @ 4.00GHz 2.6x +-+--------------------------------------------------------------------------------------------------------------+-+ | | | jr ### | 2.4x +jr+hash...........................................................................................#.#...........+-+ | # # | | # # | 2.2x +-+................................................................................................#.#...........+-+ | # # | | # # | 2x +-+................................................................................................#.#...........+-+ | **** # | | * * # | 1.8x +-+.............................................................................................*..*.#...........+-+ | +++ * * # | | #### #### * * # | 1.6x +-+......................................####.............................#..#.****..#..........*..*.#...........+-+ | +++ #++# **** # * * # #### * * # | | ### # # * * # * * # # # * * # | 1.4x +-+...................****+#..........****..#..........................*..*..#.*..*..#....#..#..*..*.#...........+-+ | *++* # * * # * * # * * # *** # * * # #### | | * * # #### * * # * * # * * # * * # * * # **** # | 1.2x +-+...................*..*.#..****++#.*..*..#..........................*..*..#.*..*..#..*.*..#..*..*.#..*..*..#..+-+ | ****### * * # * * # * * # * * # * * # * * # * * # * * # | | * * # ***### * * # * * # * * # ****## * * # * * # * * # * * # * * # | 1x +-+--****###--***###--****##--****###-****###--***###--***###--****##--****###-****###--***###--****##--****###--+-+ astar bzip2 gcc gobmk h264ref hmmlibquantum mcf omnetpperlbench sjengxalancbmk hmean png: http://imgur.com/ArCbHqo - NBench, x86_64-linux-user. Host: Intel i7-6700K @ 4.00GHz 1.12x +-+-------------------------------------------------------------------------------------------------------------+-+ | | | jr +++ | 1.1x +jr+hash...........................................................####.........................................+-+ | +++#| # | | | #++# | 1.08x +-+................................+++................+++.+++..*****..#.........................................+-+ | | +++ | | * | * # | | | | | | *+++* # | 1.06x +-+................................****###.............|...|...*...*..#.........................+++.............+-+ | *| * |# ****### * * # | | | *| *++# *| * |# * * # #### | 1.04x +-+................................*++*..#............*|.*.|#..*...*..#........................#.|#.............+-+ | * * # *++*++# * * # +++#++# | | * * # * * # * * # | # # +++#### | 1.02x +-+................................*..*..#......+++...*..*..#..*...*..#.....................****..#..*****++#...+-+ | +++ * * # +++ | * * # * * # +++ *| * # *+++* # | | +++ | +++ +++ ++++++ * * # *****### * * # * * # | +++ ++++++ *++* # * * # | 1x +-++-+++++####++****###++++-+####+-*++*++#-+*+++*-+#++*++*++#++*+-+*++#+-+++####-+*****###++*++*++#++*+-+*++#+-++-+ | *****| # *++* |# *****| # * * # * *++# * * # * * # **** |# * * # * * # * * # | | * | *| # * *++# * | *++# * * # * * # * * # * * # *| *++# * * # * * # * * # | 0.98x +-+...*.|.*++#..*..*..#..*+++*..#..*..*..#..*...*..#..*..*..#..*...*..#..*++*..#..*...*..#..*..*..#..*...*..#...+-+ | *+++* # * * # * * # * * # * * # * * # * * # * * # * * # * * # * * # | | * * # * * # * * # * * # * * # * * # * * # * * # * * # * * # * * # | 0.96x +-+---*****###--****###--*****###--****###--*****###--****###--*****###--****###--*****###--****###--*****###---+-+ ASSIGNMENT BITFIELD FOURFP EMULATION HUFFMAN LU DECOMPOSITIONEURAL NNUMERIC SOSTRING SORT hmean png: http://imgur.com/ZXFX0hJ - NBench, arm-linux-user. Host: Intel i7-4790K @ 4.00GHz 1.3x +-+-------------------------------------------------------------------------------------------------------------+-+ | #### | | jr # # +++ | 1.25x +jr+hash.....................#..#...........................................####................................+-+ | # # # # | | # # # # | 1.2x +-+..........................#..#...........................................#..#................................+-+ | # # # # | | # # # # | 1.15x +-+..........................#..#...........................................#..#................................+-+ | # # #### # # | | # # # # # # | 1.1x +-+..........................#..#..................................#..#.....#..#................................+-+ | # # # # # # +++ | | # # #### # # # # #### | 1.05x +-+..........................#..#...............#..#.....####......#..#.....#..#.........................#..#...+-+ | # # # # # # # # # # +++ # # | | +++ ***** # #### ***** # # # +++# # **** # ****### # # | 1x +-++-+*****###++****+++++*+-+*++#+-****++#-+*+++*-+#+++++#++#++*****++#+-*++*++#-+*****-++++*++*++#++*****++#+-++-+ | * * # * * | * * # * * # * * # **** # * * # * * # * *### * *++# * * # | | * * # * *### * * # * * # * * # * * # * * # * * # * * # * * # * * # | 0.95x +-+...*...*..#..*..*.|#..*...*..#..*..*..#..*...*..#..*..*..#..*...*..#..*..*..#..*...*..#..*..*..#..*...*..#...+-+ | * * # * * |# * * # * * # * * # * * # * * # * * # * * # * * # * * # | | * * # * * |# * * # * * # * * # * * # * * # * * # * * # * * # * * # | 0.9x +-+---*****###--****###--*****###--****###--*****###--****###--*****###--****###--*****###--****###--*****###---+-+ ASSIGNMENT BITFIELD FOURFP EMULATION HUFFMAN LU DECOMPOSITIONEURAL NNUMERIC SOSTRING SORT hmean png: http://imgur.com/FfD27ey Backports commit 6f1653180f5701c6a8f1b35b89a80b1e3260928e from qemu
2018-03-03 19:11:28 +00:00
#ifdef CONFIG_SOFTMMU
/* Only the bottom TB_JMP_PAGE_BITS of the jump cache hash bits vary for
addresses on the same page. The top bits are the same. This allows
TLB invalidation to quickly clear a subset of the hash table. */
#define TB_JMP_PAGE_BITS (TB_JMP_CACHE_BITS / 2)
#define TB_JMP_PAGE_SIZE (1 << TB_JMP_PAGE_BITS)
#define TB_JMP_ADDR_MASK (TB_JMP_PAGE_SIZE - 1)
#define TB_JMP_PAGE_MASK (TB_JMP_CACHE_SIZE - TB_JMP_PAGE_SIZE)
static inline unsigned int tb_jmp_cache_hash_page(target_ulong pc)
{
target_ulong tmp;
tmp = pc ^ (pc >> (TARGET_PAGE_BITS - TB_JMP_PAGE_BITS));
return (tmp >> (TARGET_PAGE_BITS - TB_JMP_PAGE_BITS)) & TB_JMP_PAGE_MASK;
}
tb-hash: improve tb_jmp_cache hash function in user mode Optimizations to cross-page chaining and indirect branches make performance more sensitive to the hit rate of tb_jmp_cache. The constraint of reserving some bits for the page number lowers the achievable quality of the hashing function. However, user-mode does not have this requirement. Thus, with this change we use for user-mode a hashing function that is both faster and of better quality than the previous one. Measurements: Note: baseline (i.e. speedup == 1x) is QEMU v2.9.0. - SPECint06 (test set), x86_64-linux-user. Host: Intel i7-6700K @ 4.00GHz 2.2x +-+--------------------------------------------------------------------------------------------------------------+-+ | | | jr | 2x +jr+multhash +....................................................+++++...................................+-+ | jr+hash |$$$ | | |$+$ | | ### $ | 1.8x +-+......................................................................#|#.$...................................+-+ | ++#+# $ | | |# # $ | 1.6x +-+....................................................................***.#.$....................++$$$..........+-+ | $$$ *+* # $ |$+$ | | ++$$$ ### $ * * # $ +++|$ $ | | ++###+$ # # $ * * # $ ### ****## $ | 1.4x +-+...................***+#.$.........***.#.$..........................*.*.#.$...........#+#$$.*++*|#.$..........+-+ | *+* # $ * * # $ * * # $ # # $ * *+# $ | | * * # $ +++++ * * # $ * * # $ *** # $ * * # $ ###$$ | 1.2x +-+...................*.*.#.$.***##$$.*.*.#.$..........................*.*.#.$.........*.*.#.$.*..*.#.$.***+#+$..+-+ | * * # $ *+* # $ * * # $ +++ * * # $ ++###$$ * * # $ * * # $ * * # $ | | ***##$$ * * # $ * * # $ * * # $ ***##$$ ++### * * # $ *** #+$ * * # $ * * # $ * * # $ | | *+*+#+$ ***##$$$ * * # $ * * # $ * * # $ *+* # $ ++####$$ ***+# * * # $ * * # $ * * # $ * * # $ * * # $ | 1x +-++-*+*+#+$+*+*+#-+$+*+*-#+$+*+*+#+$+*+*+#+$+*-*+#+$+***++#+$+*+*+#$$+*+*+#+$+*+*+#+$+*+*-#+$+*+-*+#+$+*+*+#+$-++-+ | * * # $ * * # $ * * # $ * * # $ * * # $ * * # $ * * # $ * * # $ * * # $ * * # $ * * # $ * * # $ * * # $ | | * * # $ * * # $ * * # $ * * # $ * * # $ * * # $ * * # $ * * # $ * * # $ * * # $ * * # $ * * # $ * * # $ | 0.8x +-+--***##$$-***##$$$-***##$$-***##$$-***##$$-***##$$-***###$$-***##$$-***##$$-***##$$-***##$$-****##$$-***##$$--+-+ astar bzip2 gcc gobmk h264ref hmmlibquantum mcf omnetpperlbench sjengxalancbmk hmean png: http://imgur.com/4UXTrEc Here I also tried the hash function suggested by Paolo ("multhash"): return ((uint64_t) (pc * 2654435761) >> 32) & (TB_JMP_CACHE_SIZE - 1); As you can see it is just as good as the other new function ("hash"), which is what I ended up going with. - SPECint06 (train set), x86_64-linux-user. Host: Intel i7-6700K @ 4.00GHz 2.6x +-+--------------------------------------------------------------------------------------------------------------+-+ | | | jr ### | 2.4x +jr+hash...........................................................................................#.#...........+-+ | # # | | # # | 2.2x +-+................................................................................................#.#...........+-+ | # # | | # # | 2x +-+................................................................................................#.#...........+-+ | **** # | | * * # | 1.8x +-+.............................................................................................*..*.#...........+-+ | +++ * * # | | #### #### * * # | 1.6x +-+......................................####.............................#..#.****..#..........*..*.#...........+-+ | +++ #++# **** # * * # #### * * # | | ### # # * * # * * # # # * * # | 1.4x +-+...................****+#..........****..#..........................*..*..#.*..*..#....#..#..*..*.#...........+-+ | *++* # * * # * * # * * # *** # * * # #### | | * * # #### * * # * * # * * # * * # * * # **** # | 1.2x +-+...................*..*.#..****++#.*..*..#..........................*..*..#.*..*..#..*.*..#..*..*.#..*..*..#..+-+ | ****### * * # * * # * * # * * # * * # * * # * * # * * # | | * * # ***### * * # * * # * * # ****## * * # * * # * * # * * # * * # | 1x +-+--****###--***###--****##--****###-****###--***###--***###--****##--****###-****###--***###--****##--****###--+-+ astar bzip2 gcc gobmk h264ref hmmlibquantum mcf omnetpperlbench sjengxalancbmk hmean png: http://imgur.com/ArCbHqo - NBench, x86_64-linux-user. Host: Intel i7-6700K @ 4.00GHz 1.12x +-+-------------------------------------------------------------------------------------------------------------+-+ | | | jr +++ | 1.1x +jr+hash...........................................................####.........................................+-+ | +++#| # | | | #++# | 1.08x +-+................................+++................+++.+++..*****..#.........................................+-+ | | +++ | | * | * # | | | | | | *+++* # | 1.06x +-+................................****###.............|...|...*...*..#.........................+++.............+-+ | *| * |# ****### * * # | | | *| *++# *| * |# * * # #### | 1.04x +-+................................*++*..#............*|.*.|#..*...*..#........................#.|#.............+-+ | * * # *++*++# * * # +++#++# | | * * # * * # * * # | # # +++#### | 1.02x +-+................................*..*..#......+++...*..*..#..*...*..#.....................****..#..*****++#...+-+ | +++ * * # +++ | * * # * * # +++ *| * # *+++* # | | +++ | +++ +++ ++++++ * * # *****### * * # * * # | +++ ++++++ *++* # * * # | 1x +-++-+++++####++****###++++-+####+-*++*++#-+*+++*-+#++*++*++#++*+-+*++#+-+++####-+*****###++*++*++#++*+-+*++#+-++-+ | *****| # *++* |# *****| # * * # * *++# * * # * * # **** |# * * # * * # * * # | | * | *| # * *++# * | *++# * * # * * # * * # * * # *| *++# * * # * * # * * # | 0.98x +-+...*.|.*++#..*..*..#..*+++*..#..*..*..#..*...*..#..*..*..#..*...*..#..*++*..#..*...*..#..*..*..#..*...*..#...+-+ | *+++* # * * # * * # * * # * * # * * # * * # * * # * * # * * # * * # | | * * # * * # * * # * * # * * # * * # * * # * * # * * # * * # * * # | 0.96x +-+---*****###--****###--*****###--****###--*****###--****###--*****###--****###--*****###--****###--*****###---+-+ ASSIGNMENT BITFIELD FOURFP EMULATION HUFFMAN LU DECOMPOSITIONEURAL NNUMERIC SOSTRING SORT hmean png: http://imgur.com/ZXFX0hJ - NBench, arm-linux-user. Host: Intel i7-4790K @ 4.00GHz 1.3x +-+-------------------------------------------------------------------------------------------------------------+-+ | #### | | jr # # +++ | 1.25x +jr+hash.....................#..#...........................................####................................+-+ | # # # # | | # # # # | 1.2x +-+..........................#..#...........................................#..#................................+-+ | # # # # | | # # # # | 1.15x +-+..........................#..#...........................................#..#................................+-+ | # # #### # # | | # # # # # # | 1.1x +-+..........................#..#..................................#..#.....#..#................................+-+ | # # # # # # +++ | | # # #### # # # # #### | 1.05x +-+..........................#..#...............#..#.....####......#..#.....#..#.........................#..#...+-+ | # # # # # # # # # # +++ # # | | +++ ***** # #### ***** # # # +++# # **** # ****### # # | 1x +-++-+*****###++****+++++*+-+*++#+-****++#-+*+++*-+#+++++#++#++*****++#+-*++*++#-+*****-++++*++*++#++*****++#+-++-+ | * * # * * | * * # * * # * * # **** # * * # * * # * *### * *++# * * # | | * * # * *### * * # * * # * * # * * # * * # * * # * * # * * # * * # | 0.95x +-+...*...*..#..*..*.|#..*...*..#..*..*..#..*...*..#..*..*..#..*...*..#..*..*..#..*...*..#..*..*..#..*...*..#...+-+ | * * # * * |# * * # * * # * * # * * # * * # * * # * * # * * # * * # | | * * # * * |# * * # * * # * * # * * # * * # * * # * * # * * # * * # | 0.9x +-+---*****###--****###--*****###--****###--*****###--****###--*****###--****###--*****###--****###--*****###---+-+ ASSIGNMENT BITFIELD FOURFP EMULATION HUFFMAN LU DECOMPOSITIONEURAL NNUMERIC SOSTRING SORT hmean png: http://imgur.com/FfD27ey Backports commit 6f1653180f5701c6a8f1b35b89a80b1e3260928e from qemu
2018-03-03 19:11:28 +00:00
#else
/* In user-mode we can get better hashing because we do not have a TLB */
static inline unsigned int tb_jmp_cache_hash_func(target_ulong pc)
{
return (pc ^ (pc >> TB_JMP_CACHE_BITS)) & (TB_JMP_CACHE_SIZE - 1);
}
#endif /* CONFIG_SOFTMMU */
static inline unsigned int tb_jmp_cache_hash_func(target_ulong pc)
{
target_ulong tmp;
tmp = pc ^ (pc >> (TARGET_PAGE_BITS - TB_JMP_PAGE_BITS));
return (((tmp >> (TARGET_PAGE_BITS - TB_JMP_PAGE_BITS)) & TB_JMP_PAGE_MASK)
| (tmp & TB_JMP_ADDR_MASK));
}
tb hash: hash phys_pc, pc, and flags with xxhash 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
2018-02-24 22:45:39 +00:00
static inline
uint32_t tb_hash_func(tb_page_addr_t phys_pc, target_ulong pc, uint32_t flags,
uint32_t cf_mask)
{
return qemu_xxhash7(phys_pc, pc, flags, cf_mask, 0) & (CODE_GEN_PHYS_HASH_SIZE - 1);
}
#endif