所以我从我的教授那里得到了这个问题，我不明白为什么vector2比vector1更快，缓存丢失更少。
假设下面的代码是有效的可编译C代码。
Vector2:
void incrementVector2(INT4* v, int n) {
for (int k = 0; k < 100; ++k) {
for (int i = 0; i < n; ++i) {
v[i] = v[i] + 1;
}
}
}
Vector1:
void incrementVector1(INT4* v, int n) {
for (int i = 0; i < n; ++i) {
for (int k = 0; k < 100; ++k) {
v[i] = v[i] + 1;
}
}
}
注意:
INT4表示整数大小为4字节。在CPU规格方面:缓存大小= 12288KB，行大小=64B，仅考虑此单一缓存与主存交互
为什么vector2的运行时间比vector1快?为什么vector1会比vector2有更多的缓存丢失?
我和几个同学研究了一段时间，没能弄清楚。我们认为这可能是由于vector2具有更好的空间位置性，因为数组被内循环不断访问，而在vector1中，一次只访问一个元素。我们不确定这是否正确，也不确定如何将缓存行引入其中。
我们认为这可能是由于vector2具有更好的空间位置，因为数组是由内循环访问的在vector1中，每次只访问一个元素。
好吧，这两个代码都有相同的访问模式，以1的步长迭代数组v。缓存空间位置方面，这两个代码是相同的。但是，第二个代码:
void incrementVector1(INT4* v, int n) {
for (int i = 0; i < n; ++i) {
for (int k = 0; k < 100; ++k) {
v[i] = v[i] + 1;
}
}
}
具有更好的时间局部性，因为您可以访问同一个元素100次，而在:
void incrementVector2(INT4* v, int n) {
for (int k = 0; k < 100; ++k) {
for (int i = 0; i < n; ++i) {
v[i] = v[i] + 1;
}
}
}
你只能在每'n'次迭代中访问一次。
所以要么你犯了一个错误，要么你的老师在玩某种奇怪的游戏，要么我错过了一些明显的东西。
你确定你的意思是incrementVector2更快吗?
incrementVector1具有更好的时间局部性，因此它应该更快。
并且，从基准测试来看，1比2快
当数组大小为1000时，速度快50倍。
这是令人惊讶的，因为数组足够小，可以成为[并保持]缓存热。
并且，incrementVector1的反汇编代码比incrementVector2小得多(在-O3处)。
incrementVector1为105字节，incrementVector2为203字节。
这有点令人惊讶，因为我认为尽管存在一些差异，但我没有预料到如此大的差异[见下文]。
程序输出:
0.000002238 incrementVector1
0.000019535 incrementVector2
8.729x slower
0.000000280 incrementVector1
0.000022454 incrementVector2
80.193x slower
0.000000452 incrementVector1
0.000019617 incrementVector2
43.400x slower
0.000000377 incrementVector1
0.000020632 incrementVector2
54.727x slower
0.000000361 incrementVector1
0.000022612 incrementVector2
62.637x slower
下面是我使用的程序:
#include <stdio.h>
#include <time.h>
typedef long long tsc_t;
typedef int INT4;
tsc_t
tscget(void)
{
struct timespec ts;
tsc_t tsc;
clock_gettime(CLOCK_MONOTONIC,&ts);
tsc = ts.tv_sec;
tsc *= 1000000000;
tsc += ts.tv_nsec;
return tsc;
}
double
tscsec(tsc_t tsc)
{
double sec;
sec = tsc;
sec /= 1e9;
return sec;
}
typedef void (*incfnc_p)(INT4 *v, int n);
void
incrementVector2(INT4 *v, int n)
{
for (int k = 0; k < 100; ++k) {
for (int i = 0; i < n; ++i) {
v[i] = v[i] + 1;
}
}
}
void
incrementVector1(INT4 *v, int n)
{
for (int i = 0; i < n; ++i) {
for (int k = 0; k < 100; ++k) {
v[i] = v[i] + 1;
}
}
}
INT4 v[1000] = { 0 };
#define DOFNC(_fnc)
dofnc(_fnc,#_fnc)
tsc_t
dofnc(incfnc_p fnc,const char *sym)
{
tsc_t tscbeg;
tsc_t tscend;
tscbeg = tscget();
fnc(v,sizeof(v) / sizeof(v[0]));
tscend = tscget();
tscend -= tscbeg;
printf("%.9f %sn",tscsec(tscend),sym);
return tscend;
}
void
dotest(void)
{
tsc_t tsc1 = DOFNC(incrementVector1);
tsc_t tsc2 = DOFNC(incrementVector2);
double ratio;
const char *tag;
if (tsc1 > tsc2) {
tag = "faster";
ratio = tsc1;
ratio /= tsc2;
}
else {
tag = "slower";
ratio = tsc2;
ratio /= tsc1;
}
printf("%.3fx %sn",ratio,tag);
}
int
main(void)
{
for (int testno = 1; testno <= 5; ++testno) {
printf("n");
dotest();
}
return 0;
}
下面是反汇编:
00000000004011c0 <incrementVector2>:
4011c0: 85 f6 test %esi,%esi
4011c2: 0f 8e be 00 00 00 jle 401286 <L06>
4011c8: 89 f2 mov %esi,%edx
4011ca: 41 89 f3 mov %esi,%r11d
4011cd: 66 0f 6f 0d 8b 0e 00 movdqa 0xe8b(%rip),%xmm1 # 402060 <__dso_handle+0x58>
4011d4: 00
4011d5: 49 89 f8 mov %rdi,%r8
4011d8: c1 ea 02 shr $0x2,%edx
4011db: 44 8d 56 ff lea -0x1(%rsi),%r10d
4011df: 41 83 e3 fc and $0xfffffffc,%r11d
4011e3: 41 b9 01 00 00 00 mov $0x1,%r9d
4011e9: 48 c1 e2 04 shl $0x4,%rdx
4011ed: 48 01 fa add %rdi,%rdx
4011f0:L00 41 83 fa 02 cmp $0x2,%r10d
4011f4: 0f 86 8d 00 00 00 jbe 401287 <L07>
4011fa: 48 89 f8 mov %rdi,%rax
4011fd:L01 f3 0f 6f 00 movdqu (%rax),%xmm0
401201: 48 83 c0 10 add $0x10,%rax
401205: 66 0f fe c1 paddd %xmm1,%xmm0
401209: 0f 11 40 f0 movups %xmm0,-0x10(%rax)
40120d: 48 39 d0 cmp %rdx,%rax
401210: 75 eb jne 4011fd <L01>
401212: 44 89 d8 mov %r11d,%eax
401215: 44 39 de cmp %r11d,%esi
401218: 74 22 je 40123c <L03>
40121a:L02 48 63 c8 movslq %eax,%rcx
40121d: 83 04 8f 02 addl $0x2,(%rdi,%rcx,4)
401221: 8d 48 01 lea 0x1(%rax),%ecx
401224: 39 ce cmp %ecx,%esi
401226: 7e 14 jle 40123c <L03>
401228: 48 63 c9 movslq %ecx,%rcx
40122b: 83 c0 02 add $0x2,%eax
40122e: 83 04 8f 02 addl $0x2,(%rdi,%rcx,4)
401232: 39 c6 cmp %eax,%esi
401234: 7e 06 jle 40123c <L03>
401236: 48 98 cltq
401238: 83 04 87 02 addl $0x2,(%rdi,%rax,4)
40123c:L03 41 8d 41 01 lea 0x1(%r9),%eax
401240: 41 83 c1 02 add $0x2,%r9d
401244: 41 83 f9 63 cmp $0x63,%r9d
401248: 75 a6 jne 4011f0 <L00>
40124a: be 65 00 00 00 mov $0x65,%esi
40124f: 4a 8d 7c 97 04 lea 0x4(%rdi,%r10,4),%rdi
401254: 29 c6 sub %eax,%esi
401256: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
40125d: 00 00 00
401260:L04 41 8b 10 mov (%r8),%edx
401263: 8d 42 01 lea 0x1(%rdx),%eax
401266: 01 f2 add %esi,%edx
401268: 0f 1f 84 00 00 00 00 nopl 0x0(%rax,%rax,1)
40126f: 00
401270:L05 89 c1 mov %eax,%ecx
401272: 83 c0 01 add $0x1,%eax
401275: 39 d0 cmp %edx,%eax
401277: 75 f7 jne 401270 <L05>
401279: 41 89 08 mov %ecx,(%r8)
40127c: 49 83 c0 04 add $0x4,%r8
401280: 49 39 f8 cmp %rdi,%r8
401283: 75 db jne 401260 <L04>
401285: c3 retq
401286:L06 c3 retq
401287:L07 31 c0 xor %eax,%eax
401289: eb 8f jmp 40121a <L02>
40128b: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
0000000000401290 <incrementVector1>:
401290: 85 f6 test %esi,%esi
401292: 7e 5f jle 4012f3 <L02>
401294: 8d 46 ff lea -0x1(%rsi),%eax
401297: 83 f8 02 cmp $0x2,%eax
40129a: 76 59 jbe 4012f5 <L04>
40129c: 89 f2 mov %esi,%edx
40129e: 66 0f 6f 0d ca 0d 00 movdqa 0xdca(%rip),%xmm1 # 402070 <__dso_handle+0x68>
4012a5: 00
4012a6: 48 89 f8 mov %rdi,%rax
4012a9: c1 ea 02 shr $0x2,%edx
4012ac: 48 c1 e2 04 shl $0x4,%rdx
4012b0: 48 01 fa add %rdi,%rdx
4012b3:L00 f3 0f 6f 00 movdqu (%rax),%xmm0
4012b7: 48 83 c0 10 add $0x10,%rax
4012bb: 66 0f fe c1 paddd %xmm1,%xmm0
4012bf: 0f 11 40 f0 movups %xmm0,-0x10(%rax)
4012c3: 48 39 d0 cmp %rdx,%rax
4012c6: 75 eb jne 4012b3 <L00>
4012c8: 89 f0 mov %esi,%eax
4012ca: 83 e0 fc and $0xfffffffc,%eax
4012cd: 39 f0 cmp %esi,%eax
4012cf: 74 23 je 4012f4 <L03>
4012d1:L01 48 63 d0 movslq %eax,%rdx
4012d4: 83 04 97 64 addl $0x64,(%rdi,%rdx,4)
4012d8: 8d 50 01 lea 0x1(%rax),%edx
4012db: 39 f2 cmp %esi,%edx
4012dd: 7d 14 jge 4012f3 <L02>
4012df: 48 63 d2 movslq %edx,%rdx
4012e2: 83 c0 02 add $0x2,%eax
4012e5: 83 04 97 64 addl $0x64,(%rdi,%rdx,4)
4012e9: 39 c6 cmp %eax,%esi
4012eb: 7e 06 jle 4012f3 <L02>
4012ed: 48 98 cltq
4012ef: 83 04 87 64 addl $0x64,(%rdi,%rax,4)
4012f3:L02 c3 retq
4012f4:L03 c3 retq
4012f5:L04 31 c0 xor %eax,%eax
4012f7: eb d8 jmp 4012d1 <L01>
4012f9: 0f 1f 80 00 00 00 00 nopl 0x0(%rax)
-S -fverbose-asm的输出:
.globl incrementVector2
.type incrementVector2, @function
incrementVector2:
.LFB13:
.cfi_startproc
testl %esi, %esi # n
jle .L19 #,
movl %esi, %edx # n, bnd.1
movl %esi, %r11d # n, niters_vector_mult_vf.2
movdqa .LC1(%rip), %xmm1 #, tmp152
movq %rdi, %r8 # v, ivtmp.33
shrl $2, %edx #,
leal -1(%rsi), %r10d #,
andl $-4, %r11d #, niters_vector_mult_vf.2
# orig.c:38: for (int i = 0; i < n; ++i) {
movl $1, %r9d #, ivtmp.47
salq $4, %rdx #, tmp150
addq %rdi, %rdx # v, _144
.L10:
cmpl $2, %r10d #, _72
jbe .L14 #,
# orig.c:36: {
movq %rdi, %rax # v, ivtmp.36
.L8:
# orig.c:39: v[i] = v[i] + 1;
movdqu (%rax), %xmm0 # MEM[base: _138, offset: 0B], vect__42.13
addq $16, %rax #, ivtmp.36
paddd %xmm1, %xmm0 # tmp152, vect__42.13
# orig.c:39: v[i] = v[i] + 1;
movups %xmm0, -16(%rax) # vect__42.13, MEM[base: _138, offset: 0B]
cmpq %rdx, %rax # _144, ivtmp.36
jne .L8 #,
# orig.c:38: for (int i = 0; i < n; ++i) {
movl %r11d, %eax # niters_vector_mult_vf.2, i
cmpl %r11d, %esi # niters_vector_mult_vf.2, n
je .L9 #,
.L13:
# orig.c:39: v[i] = v[i] + 1;
movslq %eax, %rcx # i, i
# orig.c:39: v[i] = v[i] + 1;
addl $2, (%rdi,%rcx,4) #, *_40
# orig.c:38: for (int i = 0; i < n; ++i) {
leal 1(%rax), %ecx #, i
# orig.c:38: for (int i = 0; i < n; ++i) {
cmpl %ecx, %esi # i, n
jle .L9 #,
# orig.c:39: v[i] = v[i] + 1;
movslq %ecx, %rcx # i, i
# orig.c:38: for (int i = 0; i < n; ++i) {
addl $2, %eax #, i
# orig.c:39: v[i] = v[i] + 1;
addl $2, (%rdi,%rcx,4) #, *_107
# orig.c:38: for (int i = 0; i < n; ++i) {
cmpl %eax, %esi # i, n
jle .L9 #,
# orig.c:39: v[i] = v[i] + 1;
cltq
# orig.c:39: v[i] = v[i] + 1;
addl $2, (%rdi,%rax,4) #, *_58
.L9:
leal 1(%r9), %eax #, _147
addl $2, %r9d #, ivtmp.47
cmpl $99, %r9d #, ivtmp.47
jne .L10 #,
movl $101, %esi #, tmp147
leaq 4(%rdi,%r10,4), %rdi #, _134
subl %eax, %esi # _147, tmp146
.p2align 4,,10
.p2align 3
.L12:
movl (%r8), %edx # MEM[base: _126, offset: 0B], _2
leal 1(%rdx), %eax #, ivtmp.21
addl %esi, %edx # tmp146, _122
.p2align 4,,10
.p2align 3
.L11:
movl %eax, %ecx # ivtmp.21, _25
addl $1, %eax #, ivtmp.21
# orig.c:38: for (int i = 0; i < n; ++i) {
cmpl %edx, %eax # _122, ivtmp.21
jne .L11 #,
movl %ecx, (%r8) # _25, MEM[base: _126, offset: 0B]
addq $4, %r8 #, ivtmp.33
# orig.c:37: for (int k = 0; k < 100; ++k) {
cmpq %rdi, %r8 # _134, ivtmp.33
jne .L12 #,
ret
.L19:
ret
.L14:
# orig.c:38: for (int i = 0; i < n; ++i) {
xorl %eax, %eax # i
jmp .L13 #
.cfi_endproc
.LFE13:
.size incrementVector2, .-incrementVector2
.p2align 4,,15
.globl incrementVector1
.type incrementVector1, @function
incrementVector1:
.LFB14:
.cfi_startproc
# orig.c:47: for (int i = 0; i < n; ++i) {
testl %esi, %esi # n
jle .L20 #,
leal -1(%rsi), %eax #, tmp118
cmpl $2, %eax #, tmp118
jbe .L27 #,
movl %esi, %edx # n, bnd.51
movdqa .LC2(%rip), %xmm1 #, tmp131
movq %rdi, %rax # v, ivtmp.62
shrl $2, %edx #,
salq $4, %rdx #, tmp121
addq %rdi, %rdx # v, _58
.L23:
movdqu (%rax), %xmm0 # MEM[base: _53, offset: 0B], vect__8.57
addq $16, %rax #, ivtmp.62
paddd %xmm1, %xmm0 # tmp131, vect__8.57
movups %xmm0, -16(%rax) # vect__8.57, MEM[base: _53, offset: 0B]
cmpq %rdx, %rax # _58, ivtmp.62
jne .L23 #,
movl %esi, %eax # n, tmp.53
andl $-4, %eax #, tmp.53
cmpl %esi, %eax # n, tmp.53
je .L29 #,
.L22:
# orig.c:49: v[i] = v[i] + 1;
movslq %eax, %rdx # tmp.53, tmp.53
addl $100, (%rdi,%rdx,4) #, *_3
# orig.c:47: for (int i = 0; i < n; ++i) {
leal 1(%rax), %edx #, i
# orig.c:47: for (int i = 0; i < n; ++i) {
cmpl %esi, %edx # n, i
jge .L20 #,
# orig.c:49: v[i] = v[i] + 1;
movslq %edx, %rdx # i, i
# orig.c:47: for (int i = 0; i < n; ++i) {
addl $2, %eax #, i
addl $100, (%rdi,%rdx,4) #, *_47
# orig.c:47: for (int i = 0; i < n; ++i) {
cmpl %eax, %esi # i, n
jle .L20 #,
# orig.c:49: v[i] = v[i] + 1;
cltq
addl $100, (%rdi,%rax,4) #, *_25
.L20:
# orig.c:52: }
ret
.L29:
ret
.L27:
# orig.c:47: for (int i = 0; i < n; ++i) {
xorl %eax, %eax # tmp.53
jmp .L22 #
.cfi_endproc
.LFE14:
.size incrementVector1, .-incrementVector1
.section .rodata.str1.1,"aMS",@progbits,1
.LC3: