On Mon, 18 Mar 2002, 20:23:48 Linus Torvalds wrote:
> On Mon, 18 Mar 2002, Linus Torvalds wrote:
> >
> > Well, I actually hink that an x86 comes fairly close.
>
> Btw, here's a program that does a simple histogram of TLB miss cost, and
> shows the interesting pattern on intel I was talking about: every 8th miss
> is most costly, apparently because Intel pre-fetches 8 TLB entries at a
> time.
>
> So on a PII core, you'll see something like
>
> 87.50: 36
> 12.39: 40
>
> ie 87.5% (exactly 7/8) of the TLB misses take 36 cycles, while 12.4% (ie
> 1/8) takes 40 cycles (and I assuem that the extra 4 cycles is due to
> actually loading the thing from the data cache).
>
> Yeah, my program might be buggy, so take the numbers with a pinch of salt.
> But it's interesting to see how on an athlon the numbers are
>
> 3.17: 59
> 34.94: 62
> 4.71: 85
> 54.83: 88
>
> ie roughly 60% take 85-90 cycles, and 40% take ~60 cycles. I don't know
> where that pattern would come from..
Linus,
it seems to be that it depends on gcc and flags.
processor : 0
vendor_id : AuthenticAMD
cpu family : 6
model : 2
model name : AMD Athlon(tm) Processor
stepping : 2
cpu MHz : 998.068
cache size : 512 KB
/home/nuetzel> gcc -v
Reading specs from /usr/lib/gcc-lib/i486-suse-linux/2.95.3/specs
gcc version 2.95.3 20010315 (SuSE)
SuSE default (-march=i486 -mcpu=i486)
/home/nuetzel> gcc -o TLB_miss TLB_miss.c
/home/nuetzel> time ./TLB_miss
12.72: 19
85.15: 21
0.460u 0.050s 0:00.50 102.0% 0+0k 0+0io 101pf+0w
/home/nuetzel> gcc -mcpu=i486 -o TLB_miss TLB_miss.c
/home/nuetzel> time ./TLB_miss
12.75: 19
84.92: 21
0.510u 0.010s 0:00.51 101.9% 0+0k 0+0io 101pf+0w
/home/nuetzel> gcc -mcpu=i686 -o TLB_miss TLB_miss.c
/home/nuetzel> time ./TLB_miss
12.96: 19
84.57: 21
0.460u 0.050s 0:00.50 102.0% 0+0k 0+0io 101pf+0w
/home/nuetzel> gcc -mcpu=k6 -o TLB_miss TLB_miss.c
/home/nuetzel> time ./TLB_miss
13.16: 19
84.88: 21
0.490u 0.010s 0:00.50 100.0% 0+0k 0+0io 101pf+0w
/home/nuetzel> gcc -O2 -mcpu=i686 -o TLB_miss TLB_miss.c
/home/nuetzel> time ./TLB_miss
2.03: 67
1.33: 80
3.50: 82
19.65: 91
1.37: 92
18.17: 93
1.59: 94
41.68: 97
2.83: 98
1.82: 106
1.60: 107
0.450u 0.000s 0:00.46 97.8% 0+0k 0+0io 101pf+0w
/home/nuetzel> gcc -O2 -mcpu=i486 -o TLB_miss TLB_miss.c
/home/nuetzel> time ./TLB_miss
1.98: 67
1.28: 80
3.37: 82
19.78: 91
1.37: 92
18.30: 93
1.59: 94
41.71: 97
2.84: 98
1.82: 106
1.60: 107
0.440u 0.010s 0:00.46 97.8% 0+0k 0+0io 101pf+0w
/home/nuetzel> gcc -O -mcpu=i486 -o TLB_miss TLB_miss.c
/home/nuetzel> time ./TLB_miss
31.73: 19
46.76: 22
9.90: 29
8.23: 30
0.430u 0.030s 0:00.45 102.2% 0+0k 0+0io 101pf+0w
/home/nuetzel> gcc -O1 -mcpu=i486 -o TLB_miss TLB_miss.c
/home/nuetzel> time ./TLB_miss
35.17: 19
47.28: 22
7.92: 29
6.70: 30
0.420u 0.040s 0:00.45 102.2% 0+0k 0+0io 101pf+0w
/home/nuetzel> gcc -Os -mcpu=i486 -o TLB_miss TLB_miss.c
/home/nuetzel> time ./TLB_miss
2.66: 67
1.79: 80
4.51: 82
18.58: 91
1.31: 92
17.11: 93
1.68: 94
40.38: 97
2.87: 98
1.80: 106
1.68: 107
0.470u 0.010s 0:00.49 97.9% 0+0k 0+0io 101pf+0w
/home/nuetzel> gcc -march=i486 -mcpu=i486 -o TLB_miss TLB_miss.c
/home/nuetzel> time ./TLB_miss
17.12: 19
80.45: 21
0.480u 0.030s 0:00.50 102.0% 0+0k 0+0io 101pf+0w
/home/nuetzel> gcc -march=i686 -mcpu=i686 -o TLB_miss TLB_miss.c
/home/nuetzel> time ./TLB_miss
17.23: 19
80.57: 21
0.480u 0.010s 0:00.50 98.0% 0+0k 0+0io 101pf+0w
/home/nuetzel> gcc -march=k6 -mcpu=k6 -o TLB_miss TLB_miss.c
/home/nuetzel> time ./TLB_miss
14.15: 19
83.81: 21
0.480u 0.030s 0:00.50 102.0% 0+0k 0+0io 101pf+0w
-- Dieter Nützel Graduate Student, Computer ScienceUniversity of Hamburg Department of Computer Science @home: Dieter.Nuetzel@hamburg.de
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