From 2a41c4b18391c2cd9094101edcdfdb6811313399 Mon Sep 17 00:00:00 2001 From: Chris Wilson Date: Thu, 10 Mar 2016 09:27:32 +0000 Subject: benchmarks/gem_syslatency: Prevent CPU sleeps (C-states) In order to keep the latency as low as possible for the idle load, we need to keep the CPU awake. Otherwise we end up with the busy workload having lower latency than the idle workload! Signed-off-by: Chris Wilson --- benchmarks/gem_syslatency.c | 14 +++++++++++++- 1 file changed, 13 insertions(+), 1 deletion(-) (limited to 'benchmarks/gem_syslatency.c') diff --git a/benchmarks/gem_syslatency.c b/benchmarks/gem_syslatency.c index 701b4362..bc513652 100644 --- a/benchmarks/gem_syslatency.c +++ b/benchmarks/gem_syslatency.c @@ -75,6 +75,16 @@ static void bind_cpu(pthread_t thread, int cpu) pthread_setaffinity_np(thread, sizeof(mask), &mask); } +static void force_low_latency(void) +{ + int32_t target = 0; + int fd = open("/dev/cpu_dma_latency", O_RDWR); + if (fd < 0 || write(fd, &target, sizeof(target)) < 0) + fprintf(stderr, + "Unable to prevent CPU sleeps and force low latency using /dev/cpu_dma_latency: %s\n", + strerror(errno)); +} + #define LOCAL_I915_EXEC_NO_RELOC (1<<11) #define LOCAL_I915_EXEC_HANDLE_LUT (1<<12) @@ -246,9 +256,11 @@ int main(int argc, char **argv) default: break; } - } + /* Prevent CPU sleeps so that busy and idle loads are consistent. */ + force_low_latency(); + busy = calloc(ncpus, sizeof(*busy)); if (enable_gem_sysbusy) { for (n = 0; n < ncpus; n++) { -- cgit v1.2.3