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Dave Hansen authored
This has been run through Intel's LKP tests across a wide range of modern sytems and workloads and it wasn't shown to make a measurable performance difference positive or negative. Now that we have some shiny new tracepoints, we can actually figure out what the heck is going on. During a kernel compile, 60% of the flush_tlb_mm_range() calls are for a single page. It breaks down like this: size percent percent<= V V V GLOBAL: 2.20% 2.20% avg cycles: 2283 1: 56.92% 59.12% avg cycles: 1276 2: 13.78% 72.90% avg cycles: 1505 3: 8.26% 81.16% avg cycles: 1880 4: 7.41% 88.58% avg cycles: 2447 5: 1.73% 90.31% avg cycles: 2358 6: 1.32% 91.63% avg cycles: 2563 7: 1.14% 92.77% avg cycles: 2862 8: 0.62% 93.39% avg cycles: 3542 9: 0.08% 93.47% avg cycles: 3289 10: 0.43% 93.90% avg cycles: 3570 11: 0.20% 94.10% avg cycles: 3767 12: 0.08% 94.18% avg cycles: 3996 13: 0.03% 94.20% avg cycles: 4077 14: 0.02% 94.23% avg cycles: 4836 15: 0.04% 94.26% avg cycles: 5699 16: 0.06% 94.32% avg cycles: 5041 17: 0.57% 94.89% avg cycles: 5473 18: 0.02% 94.91% avg cycles: 5396 19: 0.03% 94.95% avg cycles: 5296 20: 0.02% 94.96% avg cycles: 6749 21: 0.18% 95.14% avg cycles: 6225 22: 0.01% 95.15% avg cycles: 6393 23: 0.01% 95.16% avg cycles: 6861 24: 0.12% 95.28% avg cycles: 6912 25: 0.05% 95.32% avg cycles: 7190 26: 0.01% 95.33% avg cycles: 7793 27: 0.01% 95.34% avg cycles: 7833 28: 0.01% 95.35% avg cycles: 8253 29: 0.08% 95.42% avg cycles: 8024 30: 0.03% 95.45% avg cycles: 9670 31: 0.01% 95.46% avg cycles: 8949 32: 0.01% 95.46% avg cycles: 9350 33: 3.11% 98.57% avg cycles: 8534 34: 0.02% 98.60% avg cycles: 10977 35: 0.02% 98.62% avg cycles: 11400 We get in to dimishing returns pretty quickly. On pre-IvyBridge CPUs, we used to set the limit at 8 pages, and it was set at 128 on IvyBrige. That 128 number looks pretty silly considering that less than 0.5% of the flushes are that large. The previous code tried to size this number based on the size of the TLB. Good idea, but it's error-prone, needs maintenance (which it didn't get up to now), and probably would not matter in practice much. Settting it to 33 means that we cover the mallopt M_TRIM_THRESHOLD, which is the most universally common size to do flushes. That's the short version. Here's the long one for why I chose 33: 1. These numbers have a constant bias in the timestamps from the tracing. Probably counts for a couple hundred cycles in each of these tests, but it should be fairly _even_ across all of them. The smallest delta between the tracepoints I have ever seen is 335 cycles. This is one reason the cycles/page cost goes down in general as the flushes get larger. The true cost is nearer to 100 cycles. 2. A full flush is more expensive than a single invlpg, but not by much (single percentages). 3. A dtlb miss is 17.1ns (~45 cycles) and a itlb miss is 13.0ns (~34 cycles). At those rates, refilling the 512-entry dTLB takes 22,000 cycles. 4. 22,000 cycles is approximately the equivalent of doing 85 invlpg operations. But, the odds are that the TLB can actually be filled up faster than that because TLB misses that are close in time also tend to leverage the same caches. 6. ~98% of flushes are <=33 pages. There are a lot of flushes of 33 pages, probably because libc's M_TRIM_THRESHOLD is set to 128k (32 pages) 7. I've found no consistent data to support changing the IvyBridge vs. SandyBridge tunable by a factor of 16 I used the performance counters on this hardware (IvyBridge i5-3320M) to figure out the tlb miss costs: ocperf.py stat -e dtlb_load_misses.walk_duration,dtlb_load_misses.walk_completed,dtlb_store_misses.walk_duration,dtlb_store_misses.walk_completed,itlb_misses.walk_duration,itlb_misses.walk_completed,itlb.itlb_flush 7,720,030,970 dtlb_load_misses_walk_duration [57.13%] 169,856,353 dtlb_load_misses_walk_completed [57.15%] 708,832,859 dtlb_store_misses_walk_duration [57.17%] 19,346,823 dtlb_store_misses_walk_completed [57.17%] 2,779,687,402 itlb_misses_walk_duration [57.15%] 82,241,148 itlb_misses_walk_completed [57.13%] 770,717 itlb_itlb_flush [57.11%] Show that a dtlb miss is 17.1ns (~45 cycles) and a itlb miss is 13.0ns (~34 cycles). At those rates, refilling the 512-entry dTLB takes 22,000 cycles. On a SandyBridge system with more cores and larger caches, those are dtlb=13.4ns and itlb=9.5ns. cat perf.stat.txt | perl -pe 's/,//g' | awk '/itlb_misses_walk_duration/ { icyc+=$1 } /itlb_misses_walk_completed/ { imiss+=$1 } /dtlb_.*_walk_duration/ { dcyc+=$1 } /dtlb_.*.*completed/ { dmiss+=$1 } END {print "itlb cyc/miss: ", icyc/imiss, " dtlb cyc/miss: ", dcyc/dmiss, " ----- ", icyc,imiss, dcyc,dmiss } On Westmere CPUs, the counters to use are: itlb_flush,itlb_misses.walk_cycles,itlb_misses.any,dtlb_misses.walk_cycles,dtlb_misses.any The assumptions that this code went in under: https://lkml.org/lkml/2012/6/12/119 say that a flush and a refill are about 100ns. Being generous, that is over by a factor of 6 on the refill side, although it is fairly close on the cost of an invlpg. An increase of a single invlpg operation seems to lengthen the flush range operation by about 200 cycles. Here is one example of the data collected for flushing 10 and 11 pages (full data are below): 10: 0.43% 93.90% avg cycles: 3570 cycles/page: 357 samples: 4714 11: 0.20% 94.10% avg cycles: 3767 cycles/page: 342 samples: 2145 How to generate this table: echo 10000 > /sys/kernel/debug/tracing/buffer_size_kb echo x86-tsc > /sys/kernel/debug/tracing/trace_clock echo 'reason != 0' > /sys/kernel/debug/tracing/events/tlb/tlb_flush/filter echo 1 > /sys/kernel/debug/tracing/events/tlb/tlb_flush/enable Pipe the trace output in to this script: http://sr71.net/~dave/intel/201402-tlb/trace-time-diff-process.pl.txt Note that these data were gathered with the invlpg threshold set to 150 pages. Only data points with >=50 of samples were printed: Flush % of %<= in flush this pages es size ------------------------------------------------------------------------------ -1: 2.20% 2.20% avg cycles: 2283 cycles/page: xxxx samples: 23960 1: 56.92% 59.12% avg cycles: 1276 cycles/page: 1276 samples: 620895 2: 13.78% 72.90% avg cycles: 1505 cycles/page: 752 samples: 150335 3: 8.26% 81.16% avg cycles: 1880 cycles/page: 626 samples: 90131 4: 7.41% 88.58% avg cycles: 2447 cycles/page: 611 samples: 80877 5: 1.73% 90.31% avg cycles: 2358 cycles/page: 471 samples: 18885 6: 1.32% 91.63% avg cycles: 2563 cycles/page: 427 samples: 14397 7: 1.14% 92.77% avg cycles: 2862 cycles/page: 408 samples: 12441 8: 0.62% 93.39% avg cycles: 3542 cycles/page: 442 samples: 6721 9: 0.08% 93.47% avg cycles: 3289 cycles/page: 365 samples: 917 10: 0.43% 93.90% avg cycles: 3570 cycles/page: 357 samples: 4714 11: 0.20% 94.10% avg cycles: 3767 cycles/page: 342 samples: 2145 12: 0.08% 94.18% avg cycles: 3996 cycles/page: 333 samples: 864 13: 0.03% 94.20% avg cycles: 4077 cycles/page: 313 samples: 289 14: 0.02% 94.23% avg cycles: 4836 cycles/page: 345 samples: 236 15: 0.04% 94.26% avg cycles: 5699 cycles/page: 379 samples: 390 16: 0.06% 94.32% avg cycles: 5041 cycles/page: 315 samples: 643 17: 0.57% 94.89% avg cycles: 5473 cycles/page: 321 samples: 6229 18: 0.02% 94.91% avg cycles: 5396 cycles/page: 299 samples: 224 19: 0.03% 94.95% avg cycles: 5296 cycles/page: 278 samples: 367 20: 0.02% 94.96% avg cycles: 6749 cycles/page: 337 samples: 185 21: 0.18% 95.14% avg cycles: 6225 cycles/page: 296 samples: 1964 22: 0.01% 95.15% avg cycles: 6393 cycles/page: 290 samples: 83 23: 0.01% 95.16% avg cycles: 6861 cycles/page: 298 samples: 61 24: 0.12% 95.28% avg cycles: 6912 cycles/page: 288 samples: 1307 25: 0.05% 95.32% avg cycles: 7190 cycles/page: 287 samples: 533 26: 0.01% 95.33% avg cycles: 7793 cycles/page: 299 samples: 94 27: 0.01% 95.34% avg cycles: 7833 cycles/page: 290 samples: 66 28: 0.01% 95.35% avg cycles: 8253 cycles/page: 294 samples: 73 29: 0.08% 95.42% avg cycles: 8024 cycles/page: 276 samples: 846 30: 0.03% 95.45% avg cycles: 9670 cycles/page: 322 samples: 296 31: 0.01% 95.46% avg cycles: 8949 cycles/page: 288 samples: 79 32: 0.01% 95.46% avg cycles: 9350 cycles/page: 292 samples: 60 33: 3.11% 98.57% avg cycles: 8534 cycles/page: 258 samples: 33936 34: 0.02% 98.60% avg cycles: 10977 cycles/page: 322 samples: 268 35: 0.02% 98.62% avg cycles: 11400 cycles/page: 325 samples: 177 36: 0.01% 98.63% avg cycles: 11504 cycles/page: 319 samples: 161 37: 0.02% 98.65% avg cycles: 11596 cycles/page: 313 samples: 182 38: 0.02% 98.66% avg cycles: 11850 cycles/page: 311 samples: 195 39: 0.01% 98.68% avg cycles: 12158 cycles/page: 311 samples: 128 40: 0.01% 98.68% avg cycles: 11626 cycles/page: 290 samples: 78 41: 0.04% 98.73% avg cycles: 11435 cycles/page: 278 samples: 477 42: 0.01% 98.73% avg cycles: 12571 cycles/page: 299 samples: 74 43: 0.01% 98.74% avg cycles: 12562 cycles/page: 292 samples: 78 44: 0.01% 98.75% avg cycles: 12991 cycles/page: 295 samples: 108 45: 0.01% 98.76% avg cycles: 13169 cycles/page: 292 samples: 78 46: 0.02% 98.78% avg cycles: 12891 cycles/page: 280 samples: 261 47: 0.01% 98.79% avg cycles: 13099 cycles/page: 278 samples: 67 48: 0.01% 98.80% avg cycles: 13851 cycles/page: 288 samples: 77 49: 0.01% 98.80% avg cycles: 13749 cycles/page: 280 samples: 66 50: 0.01% 98.81% avg cycles: 13949 cycles/page: 278 samples: 73 52: 0.00% 98.82% avg cycles: 14243 cycles/page: 273 samples: 52 54: 0.01% 98.83% avg cycles: 15312 cycles/page: 283 samples: 87 55: 0.01% 98.84% avg cycles: 15197 cycles/page: 276 samples: 109 56: 0.02% 98.86% avg cycles: 15234 cycles/page: 272 samples: 208 57: 0.00% 98.86% avg cycles: 14888 cycles/page: 261 samples: 53 58: 0.01% 98.87% avg cycles: 15037 cycles/page: 259 samples: 59 59: 0.01% 98.87% avg cycles: 15752 cycles/page: 266 samples: 63 62: 0.00% 98.89% avg cycles: 16222 cycles/page: 261 samples: 54 64: 0.02% 98.91% avg cycles: 17179 cycles/page: 268 samples: 248 65: 0.12% 99.03% avg cycles: 18762 cycles/page: 288 samples: 1324 85: 0.00% 99.10% avg cycles: 21649 cycles/page: 254 samples: 50 127: 0.01% 99.18% avg cycles: 32397 cycles/page: 255 samples: 75 128: 0.13% 99.31% avg cycles: 31711 cycles/page: 247 samples: 1466 129: 0.18% 99.49% avg cycles: 33017 cycles/page: 255 samples: 1927 181: 0.33% 99.84% avg cycles: 2489 cycles/page: 13 samples: 3547 256: 0.05% 99.91% avg cycles: 2305 cycles/page: 9 samples: 550 512: 0.03% 99.95% avg cycles: 2133 cycles/page: 4 samples: 304 1512: 0.01% 99.99% avg cycles: 3038 cycles/page: 2 samples: 65 Here are the tlb counters during a 10-second slice of a kernel compile for a SandyBridge system. It's better than IvyBridge, but probably due to the larger caches since this was one of the 'X' extreme parts. 10,873,007,282 dtlb_load_misses_walk_duration 250,711,333 dtlb_load_misses_walk_completed 1,212,395,865 dtlb_store_misses_walk_duration 31,615,772 dtlb_store_misses_walk_completed 5,091,010,274 itlb_misses_walk_duration 163,193,511 itlb_misses_walk_completed 1,321,980 itlb_itlb_flush 10.008045158 seconds time elapsed # cat perf.stat.1392743721.txt | perl -pe 's/,//g' | awk '/itlb_misses_walk_duration/ { icyc+=$1 } /itlb_misses_walk_completed/ { imiss+=$1 } /dtlb_.*_walk_duration/ { dcyc+=$1 } /dtlb_.*.*completed/ { dmiss+=$1 } END {print "itlb cyc/miss: ", icyc/imiss/3.3, " dtlb cyc/miss: ", dcyc/dmiss/3.3, " ----- ", icyc,imiss, dcyc,dmiss }' itlb ns/miss: 9.45338 dtlb ns/miss: 12.9716 Signed-off-by:Dave Hansen <dave.hansen@linux.intel.com> Link: http://lkml.kernel.org/r/20140731154103.10C1115E@viggo.jf.intel.comAcked-by:
Rik van Riel <riel@redhat.com> Acked-by:
Mel Gorman <mgorman@suse.de> Signed-off-by:
H. Peter Anvin <hpa@linux.intel.com>
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