问题
Recently I have been observing performance effects in memory-intensive workloads I was unable to explain. Trying to get to the bottom of this I started running several microbenchmarks in order to determine common performance parameters like cache line size and L1/L2/L3 cache size (I knew them already, I just wanted to see if my measurements reflected the actual values).
For the cache line test my code roughly looks as follows (Linux C, but the concept is similiar to Windows etc. of course):
char *array = malloc (ARRAY_SIZE);
int count = ARRAY_SIZE / STEP;
clock_gettime(CLOCK_REALTIME, &start_time);
for (int i = 0; i < ARRAY_SIZE; i += STEP) {
array[i]++;
}
clock_gettime(CLOCK_REALTIME, &end_time);
// calculate time per element here:
[..]
Varying STEP from 1 to 128 shows that from STEP=64 on, I saw that the time per element did not increase further, i.e. every iteration would need to fetch a new cache line dominating the runtime.
Varying ARRAY_SIZE from 1K to 16384K keeping STEP=64 I was able to create a nice plot exhibiting a step pattern that roughly corresponds to L1, L2 and L3 latency. It was necessary to repeat the for loop a number of times, for very small array sizes even 100,000s of times, to get reliable numbers, though. Then, on my IvyBridge notebook I can clearly see L1 ending at 64K, L2 at 256K and even the L3 at 6M.
Now on to my real question: In a NUMA system, any single core will obtain remote main memory and even shared cache that is not necessarily as close as its local cache and memory. I was hoping to see a difference in latency/performance thus determining how much memory I could allocate while staying in my fast caches/part of memory.
For this, I refined my test to walk through the memory in 1/10 MB chunks measuring the latency separately and later collect the fastest chunks, roughly like this:
for (int chunk_start = 0; chunk_start < ARRAY_SIZE; chunk_start += CHUNK_SIZE) {
int chunk_end = MIN (ARRAY_SIZE, chunk_start + CHUNK_SIZE);
int chunk_els = CHUNK_SIZE / STEP;
for (int i = chunk_start; i < chunk_end; i+= STEP) {
array[i]++;
}
// calculate time per element
[..]
As soon as I start increasing ARRAY_SIZE to something larger than the L3 size, I get wildy unrealiable numbers not even a large number of repeats is able to even out. There is no way I can make out a pattern usable for performance evaluation with this, let alone determine where exactly a NUMA stripe starts, ends or is located.
Then, I figured the Hardware prefetcher is smart enough to recognize my simple access pattern and simply fetch the needed lines into the cache before I access them. Adding a random number to the array index increases the time per element but did not seem to help much otherwise, probably because I had a rand () call every iteration. Precomputing some random values and storing them in an array did not seem a good idea to me as this array as well would be stored in a hot cache and skew my measurements. Increasing STEP to 4097 or 8193 did not help much either, the prefetcher must be smarter than me.
Is my approach sensible/viable or did I miss the larger picture? Is it possible to observe NUMA latencies like this at all? If yes, what am I doing wrong? I disabled address space randomization just to be sure and preclude strange cache aliasing effects. Is there something else operating-sytem wise that has to be tuned before measuring?
回答1:
Is it possible to observe NUMA latencies like this at all? If yes, what am I doing wrong?
Memory allocators are NUMA aware, so by default you will not observe any NUMA effects until you explicitly ask to allocate memory on another node. The most simple way to achieve the effect is numactl(8). Just run your application on one node and bind memory allocations to another, like so:
numactl --cpunodebind 0 --membind 1 ./my-benchmark
See also numa_alloc_onnode(3).
Is there something else operating-sytem wise that has to be tuned before measuring?
Turn off CPU scaling otherwise your measurements might be noisy:
find '/sys/devices/system/cpu/' -name 'scaling_governor' | while read F; do
echo "==> ${F}"
echo "performance" | sudo tee "${F}" > /dev/null
done
Now regarding the test itself. Sure, to measure the latency, access pattern must be (pseudo) random. Otherwise your measurements will be contaminated with fast cache hits.
Here is an example how you could achieve this:
Data Initialization
Fill the array with random numbers:
static void random_data_init()
{
for (size_t i = 0; i < ARR_SZ; i++) {
arr[i] = rand();
}
}
Benchmark
Perform 1M op operations per one benchmark iteration to reduce measurement noise. Use array random number to jump over few cache lines:
const size_t OPERATIONS = 1 * 1000 * 1000; // 1M operations per iteration
int random_step_sizeK(size_t size)
{
size_t idx = 0;
for (size_t i = 0; i < OPERATIONS; i++) {
arr[idx & (size - 1)]++;
idx += arr[idx & (size - 1)] * 64; // assuming cache line is 64B
}
return 0;
}
Results
Here are the results for i5-4460 CPU @ 3.20GHz:
----------------------------------------------------------------
Benchmark Time CPU Iterations
----------------------------------------------------------------
random_step_sizeK/4 4217004 ns 4216880 ns 166
random_step_sizeK/8 4146458 ns 4146227 ns 168
random_step_sizeK/16 4188168 ns 4187700 ns 168
random_step_sizeK/32 4180545 ns 4179946 ns 163
random_step_sizeK/64 5420788 ns 5420140 ns 129
random_step_sizeK/128 6187776 ns 6187337 ns 112
random_step_sizeK/256 7856840 ns 7856549 ns 89
random_step_sizeK/512 11311684 ns 11311258 ns 57
random_step_sizeK/1024 13634351 ns 13633856 ns 51
random_step_sizeK/2048 16922005 ns 16921141 ns 48
random_step_sizeK/4096 15263547 ns 15260469 ns 41
random_step_sizeK/6144 15262491 ns 15260913 ns 46
random_step_sizeK/8192 45484456 ns 45482016 ns 23
random_step_sizeK/16384 54070435 ns 54064053 ns 14
random_step_sizeK/32768 59277722 ns 59273523 ns 11
random_step_sizeK/65536 63676848 ns 63674236 ns 10
random_step_sizeK/131072 66383037 ns 66380687 ns 11
There are obvious steps between 32K/64K (so my L1 cache is ~32K), 256K/512K (so my L2 cache size is ~256K) and 6144K/8192K (so my L3 cache size is ~6M).
来源：https://stackoverflow.com/questions/47749905/determine-numa-layout-via-latency-performance-measurements