Significant FMA performance anomaly experienced in the Intel Broadwell processor
Code1:
vzeroall mov rcx, 1000000 startLabel1: vfmadd231ps ymm0, ymm0, ymm0 vfmadd231ps ymm1, ymm1, ymm1 vfmadd231ps ymm2, ym
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Updated
I've got no explanation for you, since I'm on Haswell, but I do have code to share that might help you or someone else with Broadwell or Skylake hardware isolate your problem. If you could please run it on your machine and share the results, we could gain an insight into what's happening to your machine.
Intro
Recent Intel Core i7 processors have 7 performance monitor counters (PMCs), 3 fixed-function and 4 general-purpose, that may be used to profile code. The fixed-function PMCs are:
- Instructions retired
- Unhalted core cycles (Clock ticks including the effects of TurboBoost)
- Unhalted Reference cycles (Fixed-frequency clock ticks)
The ratio of core:reference clock cycles determines the relative speedup or slowdown from dynamic frequency scaling.
Although software exists (see comments below) that accesses these counters, I did not know them and still find them to be insufficiently fine-grained.
I therefore wrote myself a Linux kernel module,
perfcount, over the past few days to grant me access to the Intel performance counter monitors, and a userspace testbench and library for your code that wraps your FMA code around calls to my LKM. Instructions for how to reproduce my setup will follow.My testbench source code is below. It warms up, then runs your code several times, testing it over a long list of metrics. I changed your loop count to 1 billion. Because only 4 general-purpose PMCs can be programmed at once, I do the measurements 4 at a time.
perfcountdemo.c/* Includes */ #include "libperfcount.h" #include <ctype.h> #include <stdint.h> #include <stdio.h> #include <stdlib.h> #include <string.h> /* Function prototypes */ void code1(void); void code2(void); void code3(void); void code4(void); void code5(void); /* Global variables */ void ((*FN_TABLE[])(void)) = { code1, code2, code3, code4, code5 }; /** * Code snippets to bench */ void code1(void){ asm volatile( ".intel_syntax noprefix\n\t" "vzeroall\n\t" "mov rcx, 1000000000\n\t" "LstartLabel1:\n\t" "vfmadd231ps %%ymm0, %%ymm0, %%ymm0\n\t" "vfmadd231ps ymm1, ymm1, ymm1\n\t" "vfmadd231ps ymm2, ymm2, ymm2\n\t" "vfmadd231ps ymm3, ymm3, ymm3\n\t" "vfmadd231ps ymm4, ymm4, ymm4\n\t" "vfmadd231ps ymm5, ymm5, ymm5\n\t" "vfmadd231ps ymm6, ymm6, ymm6\n\t" "vfmadd231ps ymm7, ymm7, ymm7\n\t" "vfmadd231ps ymm8, ymm8, ymm8\n\t" "vfmadd231ps ymm9, ymm9, ymm9\n\t" "vpaffffd ymm10, ymm10, ymm10\n\t" "vpaffffd ymm11, ymm11, ymm11\n\t" "vpaffffd ymm12, ymm12, ymm12\n\t" "vpaffffd ymm13, ymm13, ymm13\n\t" "vpaffffd ymm14, ymm14, ymm14\n\t" "dec rcx\n\t" "jnz LstartLabel1\n\t" ".att_syntax noprefix\n\t" : /* No outputs we care about */ : /* No inputs we care about */ : "xmm0", "xmm1", "xmm2", "xmm3", "xmm4", "xmm5", "xmm6", "xmm7", "xmm8", "xmm9", "xmm10", "xmm11", "xmm12", "xmm13", "xmm14", "xmm15", "rcx", "memory" ); } void code2(void){ } void code3(void){ } void code4(void){ } void code5(void){ } /* Test Schedule */ const char* const SCHEDULE[] = { /* Batch */ "uops_issued.any", "uops_issued.any<1", "uops_issued.any>=1", "uops_issued.any>=2", /* Batch */ "uops_issued.any>=3", "uops_issued.any>=4", "uops_issued.any>=5", "uops_issued.any>=6", /* Batch */ "uops_executed_port.port_0", "uops_executed_port.port_1", "uops_executed_port.port_2", "uops_executed_port.port_3", /* Batch */ "uops_executed_port.port_4", "uops_executed_port.port_5", "uops_executed_port.port_6", "uops_executed_port.port_7", /* Batch */ "resource_stalls.any", "resource_stalls.rs", "resource_stalls.sb", "resource_stalls.rob", /* Batch */ "uops_retired.all", "uops_retired.all<1", "uops_retired.all>=1", "uops_retired.all>=2", /* Batch */ "uops_retired.all>=3", "uops_retired.all>=4", "uops_retired.all>=5", "uops_retired.all>=6", /* Batch */ "inst_retired.any_p", "inst_retired.any_p<1", "inst_retired.any_p>=1", "inst_retired.any_p>=2", /* Batch */ "inst_retired.any_p>=3", "inst_retired.any_p>=4", "inst_retired.any_p>=5", "inst_retired.any_p>=6", /* Batch */ "idq_uops_not_delivered.core", "idq_uops_not_delivered.core<1", "idq_uops_not_delivered.core>=1", "idq_uops_not_delivered.core>=2", /* Batch */ "idq_uops_not_delivered.core>=3", "idq_uops_not_delivered.core>=4", "rs_events.empty", "idq.empty", /* Batch */ "idq.mite_all_uops", "idq.mite_all_uops<1", "idq.mite_all_uops>=1", "idq.mite_all_uops>=2", /* Batch */ "idq.mite_all_uops>=3", "idq.mite_all_uops>=4", "move_elimination.int_not_eliminated", "move_elimination.simd_not_eliminated", /* Batch */ "lsd.uops", "lsd.uops<1", "lsd.uops>=1", "lsd.uops>=2", /* Batch */ "lsd.uops>=3", "lsd.uops>=4", "ild_stall.lcp", "ild_stall.iq_full", /* Batch */ "br_inst_exec.all_branches", "br_inst_exec.0x81", "br_inst_exec.0x82", "icache.misses", /* Batch */ "br_misp_exec.all_branches", "br_misp_exec.0x81", "br_misp_exec.0x82", "fp_assist.any", /* Batch */ "cpu_clk_unhalted.core_clk", "cpu_clk_unhalted.ref_xclk", "baclears.any" }; const int NUMCOUNTS = sizeof(SCHEDULE)/sizeof(*SCHEDULE); /** * Main */ int main(int argc, char* argv[]){ int i; /** * Initialize */ pfcInit(); if(argc <= 1){ pfcDumpEvents(); exit(1); } pfcPinThread(3); /** * Arguments are: * * perfcountdemo #codesnippet * * There is a schedule of configuration that is followed. */ void (*fn)(void) = FN_TABLE[strtoull(argv[1], NULL, 0)]; static const uint64_t ZERO_CNT[7] = {0,0,0,0,0,0,0}; static const uint64_t ZERO_CFG[7] = {0,0,0,0,0,0,0}; uint64_t cnt[7] = {0,0,0,0,0,0,0}; uint64_t cfg[7] = {2,2,2,0,0,0,0}; /* Warmup */ for(i=0;i<10;i++){ fn(); } /* Run master loop */ for(i=0;i<NUMCOUNTS;i+=4){ /* Configure counters */ const char* sched0 = i+0 < NUMCOUNTS ? SCHEDULE[i+0] : ""; const char* sched1 = i+1 < NUMCOUNTS ? SCHEDULE[i+1] : ""; const char* sched2 = i+2 < NUMCOUNTS ? SCHEDULE[i+2] : ""; const char* sched3 = i+3 < NUMCOUNTS ? SCHEDULE[i+3] : ""; cfg[3] = pfcParseConfig(sched0); cfg[4] = pfcParseConfig(sched1); cfg[5] = pfcParseConfig(sched2); cfg[6] = pfcParseConfig(sched3); pfcWrConfigCnts(0, 7, cfg); pfcWrCountsCnts(0, 7, ZERO_CNT); pfcRdCountsCnts(0, 7, cnt); /* ^ Should report 0s, and launch the counters. */ /************** Hot section **************/ fn(); /************ End Hot section ************/ pfcRdCountsCnts(0, 7, cnt); pfcWrConfigCnts(0, 7, ZERO_CFG); /* ^ Should clear the counter config and disable them. */ /** * Print the lovely results */ printf("Instructions Issued : %20llu\n", cnt[0]); printf("Unhalted core cycles : %20llu\n", cnt[1]); printf("Unhalted reference cycles : %20llu\n", cnt[2]); printf("%-35s: %20llu\n", sched0, cnt[3]); printf("%-35s: %20llu\n", sched1, cnt[4]); printf("%-35s: %20llu\n", sched2, cnt[5]); printf("%-35s: %20llu\n", sched3, cnt[6]); } /** * Close up shop */ pfcFini(); }On my machine, I got the following results:
Haswell Core i7-4700MQ
> ./perfcountdemo 0 Instructions Issued : 17000001807 Unhalted core cycles : 5305920785 Unhalted reference cycles : 4245764952 uops_issued.any : 16000811079 uops_issued.any<1 : 1311417889 uops_issued.any>=1 : 4000292290 uops_issued.any>=2 : 4000229358 Instructions Issued : 17000001806 Unhalted core cycles : 5303822082 Unhalted reference cycles : 4243345896 uops_issued.any>=3 : 4000156998 uops_issued.any>=4 : 4000110067 uops_issued.any>=5 : 0 uops_issued.any>=6 : 0 Instructions Issued : 17000001811 Unhalted core cycles : 5314227923 Unhalted reference cycles : 4252020624 uops_executed_port.port_0 : 5016261477 uops_executed_port.port_1 : 5036728509 uops_executed_port.port_2 : 5282 uops_executed_port.port_3 : 12481 Instructions Issued : 17000001816 Unhalted core cycles : 5329351248 Unhalted reference cycles : 4265809728 uops_executed_port.port_4 : 7087 uops_executed_port.port_5 : 4946019835 uops_executed_port.port_6 : 1000228324 uops_executed_port.port_7 : 1372 Instructions Issued : 17000001816 Unhalted core cycles : 5325153463 Unhalted reference cycles : 4261060248 resource_stalls.any : 1322734589 resource_stalls.rs : 844250210 resource_stalls.sb : 0 resource_stalls.rob : 0 Instructions Issued : 17000001814 Unhalted core cycles : 5327823817 Unhalted reference cycles : 4262914728 uops_retired.all : 16000445793 uops_retired.all<1 : 687284798 uops_retired.all>=1 : 4646263984 uops_retired.all>=2 : 4452324050 Instructions Issued : 17000001809 Unhalted core cycles : 5311736558 Unhalted reference cycles : 4250015688 uops_retired.all>=3 : 3545695253 uops_retired.all>=4 : 3341664653 uops_retired.all>=5 : 1016 uops_retired.all>=6 : 1 Instructions Issued : 17000001871 Unhalted core cycles : 5477215269 Unhalted reference cycles : 4383891984 inst_retired.any_p : 17000001871 inst_retired.any_p<1 : 891904306 inst_retired.any_p>=1 : 4593972062 inst_retired.any_p>=2 : 4441024510 Instructions Issued : 17000001835 Unhalted core cycles : 5377202052 Unhalted reference cycles : 4302895152 inst_retired.any_p>=3 : 3555852364 inst_retired.any_p>=4 : 3369559466 inst_retired.any_p>=5 : 999980244 inst_retired.any_p>=6 : 0 Instructions Issued : 17000001826 Unhalted core cycles : 5349373678 Unhalted reference cycles : 4280991912 idq_uops_not_delivered.core : 1580573 idq_uops_not_delivered.core<1 : 5354931839 idq_uops_not_delivered.core>=1 : 471248 idq_uops_not_delivered.core>=2 : 418625 Instructions Issued : 17000001808 Unhalted core cycles : 5309687640 Unhalted reference cycles : 4248083976 idq_uops_not_delivered.core>=3 : 280800 idq_uops_not_delivered.core>=4 : 247923 rs_events.empty : 0 idq.empty : 649944 Instructions Issued : 17000001838 Unhalted core cycles : 5392229041 Unhalted reference cycles : 4315704216 idq.mite_all_uops : 2496139 idq.mite_all_uops<1 : 5397877484 idq.mite_all_uops>=1 : 971582 idq.mite_all_uops>=2 : 595973 Instructions Issued : 17000001822 Unhalted core cycles : 5347205506 Unhalted reference cycles : 4278845208 idq.mite_all_uops>=3 : 394011 idq.mite_all_uops>=4 : 335205 move_elimination.int_not_eliminated: 0 move_elimination.simd_not_eliminated: 0 Instructions Issued : 17000001812 Unhalted core cycles : 5320621549 Unhalted reference cycles : 4257095280 lsd.uops : 15999287982 lsd.uops<1 : 1326629729 lsd.uops>=1 : 3999821996 lsd.uops>=2 : 3999821996 Instructions Issued : 17000001813 Unhalted core cycles : 5320533147 Unhalted reference cycles : 4257105096 lsd.uops>=3 : 3999823498 lsd.uops>=4 : 3999823498 ild_stall.lcp : 0 ild_stall.iq_full : 3468 Instructions Issued : 17000001813 Unhalted core cycles : 5323278281 Unhalted reference cycles : 4258969200 br_inst_exec.all_branches : 1000016626 br_inst_exec.0x81 : 1000016616 br_inst_exec.0x82 : 0 icache.misses : 294 Instructions Issued : 17000001812 Unhalted core cycles : 5315098728 Unhalted reference cycles : 4253082504 br_misp_exec.all_branches : 5 br_misp_exec.0x81 : 2 br_misp_exec.0x82 : 0 fp_assist.any : 0 Instructions Issued : 17000001819 Unhalted core cycles : 5338484610 Unhalted reference cycles : 4271432976 cpu_clk_unhalted.core_clk : 5338494250 cpu_clk_unhalted.ref_xclk : 177976806 baclears.any : 1 : 0We may see that on Haswell, everything is well-oiled. I'll make a few notes from the above stats:
- Instructions issued is incredibly consistent for me. It's always around
17000001800, which is a good sign: It means we can make a very good estimate of our overhead. Idem for the other fixed-function counters. The fact that they all match reasonably well means that the tests in batches of 4 are apples-to-apples comparisons. - With a ratio of core:reference cycles of around 5305920785/4245764952, we get an average frequency scaling of ~1.25; This jives well with my observations that my core clocked up from 2.4 GHz to 3.0 GHz.
cpu_clk_unhalted.core_clk/(10.0*cpu_clk_unhalted.ref_xclk)gives just under 3 GHz too. - The ratio of instructions issued to core cycles gives the IPC, 17000001807/5305920785 ~ 3.20, which is also about right: 2 FMA+1 VPADDD every clock cycle for 4 clock cycles, and 2 extra loop control instructions every 5th clock cycle that go in parallel.
uops_issued.any: The number of instructions issued is ~17B, but the number of uops issued is ~16B. That's because the two instructions for loop control are fusing together; Good sign. Moreover, around 1.3B clock cycles out of 5.3B (25% of the time), no uops were issued, while the near-totality of the rest of the time (4B clock cycles), 4 uops issued at a time.uops_executed_port.port_[0-7]: Port saturation. We're in good health. Of the 16B post-fusion uops, Ports 0, 1 and 5 ate 5B uops each over 5.3B cycles (Which means they were distributed optimally: Float, float, int respectively), Port 6 ate 1B (the fused dec-branch op), and ports 2, 3, 4 and 7 ate negligible amounts by comparison.resource_stalls: 1.3B of them occurred, 2/3 of which were due to the reservation station (RS) and the other third to unknown causes.- From the cumulative distribution we built with our comparisons on
uops_retired.allandinst_retired.all, we know we are retiring 4 uops 60% of the time, 0 uops 13% of the time and 2 uops the rest of the time, with negligible amounts otherwise. - (Numerous
*idq*counts): The IDQ only rarely holds us up. lsd: The Loop Stream Detector is working; Nearly 16B fused uops were supplied to the frontend from it.ild: Instruction length decoding is not the bottleneck, and not a single length-changing prefix is encountered.br_inst_exec/br_misp_exec: Branch misprediction is a negligible problem.icache.misses: Negligible.fp_assist: Negligible. Denormals not encountered. (I believe that without DAZ denormals-are-zero flushing, they'd require an assist, which should register here)
So on Intel Haswell it's smooth sailing. If you could run my suite on your machines, that would be great.
Instructions for Reproduction
- Rule #1: Inspect all my code before doing anything with it. Never blindly trust strangers on the Internet.
- Grab perfcountdemo.c, libperfcount.c and libperfcount.h, put them in the same directory and compile them together.
- Grab perfcount.c and Makefile, put them in the same directory, and
makethe kernel module. - Reboot your machine with the GRUB boot flags
nmi_watchdog=0 modprobe.blacklist=iTCO_wdt,iTCO_vendor_support. The NMI watchdog will tamper with the unhalted-core-cycle counter otherwise. insmod perfcount.kothe module.dmesg | tail -n 10should say it successfully loaded and say there are 3 Ff counters and 4 Gp counters, or else give a reason for failing to do so.- Run my application, preferably while the rest of the system is not under load. Try also changing in
perfcountdemo.cthe core to which you restrict your affinity by changing the argument topfcPinThread(). - Edit in here the results.
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Update: previous version contained a 6
VPADDDinstructions (vs 5 in the question), and the extraVPADDDcaused imbalance on Broadwell. After it was fixed, Haswell, Broadwell and Skylake issue almost the same number of uops to ports 0, 1 and 5.There is no port contamination, but uops are scheduled suboptimally, with the majority of uops going to Port 5 on Broadwell, and making it the bottleneck before Ports 0 and 1 are saturated.To demonstrate what is going on, I suggest to (ab)use the demo on PeachPy.IO:
Open www.peachpy.io in Google Chrome (it wouldn't work in other browsers).
Replace the default code (which implements SDOT function) with the code below, which is literally your example ported to PeachPy syntax:
n = Argument(size_t) x = Argument(ptr(const_float_)) incx = Argument(size_t) y = Argument(ptr(const_float_)) incy = Argument(size_t) with Function("sdot", (n, x, incx, y, incy)) as function: reg_n = GeneralPurposeRegister64() LOAD.ARGUMENT(reg_n, n) VZEROALL() with Loop() as loop: for i in range(15): ymm_i = YMMRegister(i) if i < 10: VFMADD231PS(ymm_i, ymm_i, ymm_i) else: VPADDD(ymm_i, ymm_i, ymm_i) DEC(reg_n) JNZ(loop.begin) RETURN()I have a number of machines on different microarchitectures as a backend for PeachPy.io. Choose Intel Haswell, Intel Broadwell, or Intel Skylake and press "Quick Run". The system will compile your code, upload it to server, and visualize performance counters collected during execution.
Here is the uops distribution over execution ports on Intel Haswell:
- And here is the same plot from Intel Broadwell:
- Apparently, whatever was the flaw in uops scheduler, it was fixed in Intel Skylake, because port pressure on that machine is the same as on Haswell.
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