Tutorial :How does one write code that best utilizes the CPU cache to improve performance?



Question:

This could sound like a subjective question, but what I am looking for are specific instances, which you could have encountered related to this.

  1. How to make code, cache effective/cache friendly (more cache hits, as few cache misses as possible)? From both perspectives, data cache & program cache (instruction cache), i.e. what things in one's code, related to data structures and code constructs, should one take care of to make it cache effective.

  2. Are there any particular data structures one must use/avoid, or is there a particular way of accessing the members of that structure etc... to make code cache effective.

  3. Are there any program constructs (if, for, switch, break, goto,...), code-flow (for inside an if, if inside a for, etc ...) one should follow/avoid in this matter?

I am looking forward to hearing individual experiences related to making cache efficient code in general. It can be any programming language (C, C++, Assembly, ...), any hardware target (ARM, Intel, PowerPC, ...), any OS (Windows, Linux,S ymbian, ...), etc..

The variety will help to better to understand it deeply.


Solution:1

The cache is there to reduce the number of times the CPU would stall waiting for a memory request to be fulfilled (avoiding the memory latency), and as a second effect, possibly to reduce the overall amount of data that needs to be transfered (preserving memory bandwidth).

Techniques for avoiding suffering from memory fetch latency is typically the first thing to consider, and sometimes helps a long way. The limited memory bandwidth is also a limiting factor, particularly for multicores and multithreaded applications where many threads wants to use the memory bus. A different set of techniques help addressing the latter issue.

Improving spatial locality means that you ensure that each cache line is used in full once it has been mapped to a cache. When we have looked at various standard benchmarks, we have seen that a surprising large fraction of those fail to use 100% of the fetched cache lines before the cache lines are evicted.

Improving cache line utilization helps in three respects:

  • It tends to fit more useful data in the cache, essentially increasing the effective cache size.
  • It tends to fit more useful data in the same cache line, increasing the likelyhood that requested data can be found in the cache.
  • It reduces the memory bandwidth requirements, as there will be fewer fetches.

Common techniques are:

  • Use smaller data types
  • Organize your data to avoid alignment holes (sorting your struct members by decreasing size is one way)
  • Beware of the standard dynamic memory allocator, which may introduce holes and spread your data around in memory as it warms up.
  • Make sure all adjacent data is actually used in the hot loops. Otherwise, consider breaking up data structures into hot and cold components, so that the hot loops use hot data.
  • avoid algorithms and datastructures that exhibit irregular access patterns, and favor linear datastructures.

We should also note that there are other ways to hide memory latency than using caches.

Modern CPU:s often have one or more hardware prefetchers. They train on the misses in a cache and try to spot regularities. For instance, after a few misses to subsequent cache lines, the hw prefetcher will start fetching cache lines into the cache, anticipating the application's needs. If you have a regular access pattern, the hardware prefetcher is usually doing a very good job. And if your program doesn't display regular access patterns, you may improve things by adding prefetch instructions yourself.

Regrouping instructions in such a way that those that always miss in the cache occur close to each other, the CPU can sometimes overlap these fetches so that the application only sustain one latency hit (Memory level parallelism).

To reduce the overall memory bus pressure, you have to start addressing what is called temporal locality. This means that you have to reuse data while it still hasn't been evicted from the cache.

Merging loops that touch the same data (loop fusion), and employing rewriting techniques known as tiling or blocking all strive to avoid those extra memory fetches.

While there are some rules of thumb for this rewrite exercise, you typically have to carefully consider loop carried data dependencies, to ensure that you don't affect the semantics of the program.

These things are what really pays off in the multicore world, where you typically wont see much of throughput improvements after adding the second thread.


Solution:2

I can't believe there aren't more answers to this. Anyway, one classic example is to iterate a multidimensional array "inside out":

pseudocode  for (i = 0 to size)    for (j = 0 to size)      do something with ary[j][i]  

The reason this is cache inefficient is because modern CPUs will load the cache line with "near" memory addresses from main memory when you access a single memory address. We are iterating through the "j" (outer) rows in the array in the inner loop, so for each trip through the inner loop, the cache line will cause to be flushed and loaded with a line of addresses that are near to the [j][i] entry. If this is changed to the equivalent:

for (i = 0 to size)    for (j = 0 to size)      do something with ary[i][j]  

It will run much faster.


Solution:3

I recommend reading the 9-part article What every programmer should know about memory by Ulrich Drepper if you're interested in how memory and software interact. It's also available as a 104-page PDF.

Sections especially relevant to this question might be Part 2 (CPU caches) and Part 5 (What programmers can do - cache optimization).


Solution:4

The basic rules are actually fairly simple. Where it gets tricky is in how they apply to your code.

The cache works on two principles: Temporal locality and spatial locality. The former is the idea that if you recently used a certain chunk of data, you'll probably need it again soon. The latter means that if you recently used the data at address X, you'll probably soon need address X+1.

The cache tries to accomodate this by remembering the most recently used chunks of data. It operates with cache lines, typically sized 128 byte or so, so even if you only need a single byte, the entire cache line that contains it gets pulled into the cache. So if you need the following byte afterwards, it'll already be in the cache.

And this means that you'll always want your own code to exploit these two forms of locality as much as possible. Don't jump all over memory. Do as much work as you can on one small area, and then move on to the next, and do as much work there as you can.

A simple example is the 2D array traversal that 1800's answer showed. If you traverse it a row at a time, you're reading the memory sequentially. If you do it column-wise, you'll read one entry, then jump to a completely different location (the start of the next row), read one entry, and jump again. And when you finally get back to the first row, it will no longer be in the cache.

The same applies to code. Jumps or branches mean less efficient cache usage (because you're not reading the instructions sequentially, but jumping to a different address). Of course, small if-statements probably won't change anything (you're only skipping a few bytes, so you'll still end up inside the cached region), but function calls typically imply that you're jumping to a completely different address that may not be cached. Unless it was called recently.

Instruction cache usage is usually far less of an issue though. What you usually need to worry about is the data cache.

In a struct or class, all members are laid out contiguously, which is good. In an array, all entries are laid out contiguously as well. In linked lists, each node is allocated at a completely different location, which is bad. Pointers in general tend to point to unrelated addresses, which will probably result in a cache miss if you dereference it.

And if you want to exploit multiple cores, it can get really interesting, as usually, only one CPU may have any given address in its L1 cache at a time. So if both cores constantly access the same address, it will result in constant cache misses, as they're fighting over the address.


Solution:5

Apart from data access patterns, a major factor in cache-friendly code is data size. Less data means more of it fits into the cache.

This is mainly a factor with memory-aligned data structures. "Conventional" wisdom says data structures must be aligned at word boundaries because the CPU can only access entire words, and if a word contains more than one value, you have to do extra work (read-modify-write instead of a simple write). But caches can completely invalidate this argument.

Similarly, a Java boolean array uses an entire byte for each value in order to allow operating on individual values directly. You can reduce the data size by a factor of 8 if you use actual bits, but then access to individual values becomes much more complex, requiring bit shift and mask operations (the BitSet class does this for you). However, due to cache effects, this can still be considerably faster than using a boolean[] when the array is large. IIRC I once achieved a speedup by a factor of 2 or 3 this way.


Solution:6

The most effective data structure for a cache is an array. Caches work best, if your data structure is laid out sequentially as CPUs read entire cache lines (usually 32 bytes or more) at once from main memory.

Any algorithm which accesses memory in random order trashes the caches because it always needs new cache lines to accomodate the randomly accessed memory. On the other hand an algorithm, which runs sequentially through an array is best because:

  1. It gives the CPU a chance to read-ahead, e.g. speculatively put more memory into the cache, which will be accessed later. This read-ahead gives a huge performance boost.

  2. Running a tight loop over a large array also allows the CPU to cache the code executing in the loop and in most cases allows you to execute an algorithm entirely from cache memory without having to block for external memory access.


Solution:7

A remark to the "classic example" by user 1800 INFORMATION (too long for a comment)

I wanted to check the time differences for two iteration orders ( "outter" and "inner"), so I made a simple experiment with a large 2D array:

measure::start();  for ( int y = 0; y < N; ++y )  for ( int x = 0; x < N; ++x )      sum += A[ x + y*N ];  measure::stop();  

and the second case with the for loops swapped.

The slower version ("x first") was 0.88sec and the faster one, was 0.06sec. That's the power of caching :)

I used gcc -O2 and still the loops were not optimized out. The comment by Ricardo that "most of the modern compilers can figure this out by itselves" does not hold


Solution:8

Only one post touched on it, but a big issue comes up when sharing data between processes. You want to avoid having multiple processes attempting to modify the same cache line simultaneously. Something to look out for here is "false" sharing, where two adjacent data structures share a cache line and modifications to one invalidates the cache line for the other. This can cause cache lines to unnecessarily move back and forth between processor caches sharing the data on a multiprocessor system. A way to avoid it is to align and pad data structures to put them on different lines.


Solution:9

One example I saw used in a game engine was to move data out of objects and into their own arrays. A game object that was subject to physics might have a lot of other data attached to it as well. But during the physics update loop all the engine cared about was data about position, speed, mass, bounding box, etc. So all of that was placed into its own arrays and optimized as much as possible for SSE.

So during the physics loop the physics data was processed in array order using vector math. The game objects used their object ID as the index into the various arrays. It was not a pointer because pointers could become invalidated if the arrays had to be relocated.

In many ways this violated object-oriented design patterns but it made the code a lot faster by placing data close together that needed to be operated on in the same loops.

This example is probably out of date because I expect most modern games use a prebuilt physics engine like Havok.


Solution:10

I can answer (2) by saying that in the C++ world, linked lists can easily kill the CPU cache. Arrays are a better solution where possible. No experience on whether the same applies to other languages, but it's easy to imagine the same issues would arise.


Solution:11

Cache is arranged in "cache lines" and (real) memory is read from and written to in chunks of this size.

Data structures that are contained within a single cache-line are therefore more efficient.

Similarly, algorithms which access contiguous memory blocks will be more efficient than algorithms which jump through memory in a random order.

Unfortunately the cache line size varies dramatically between processors, so there's no way to guarantee that a data structure that's optimal on one processor will be efficient on any other.


Solution:12

To ask how to make a code, cache effective-cache friendly and most of the other questions , is usually to ask how to Optimize a program, that's because the cache has such a huge impact on performances that any optimized program is one that is cache effective-cache friendly.

I suggest reading about Optimization, there are some good answers on this site. In terms of books, I recommend on Computer Systems: A Programmer's Perspective which has some fine text about the proper usage of the cache.

(b.t.w - as bad as a cache-miss can be, there is worse - if a program is paging from the hard-drive...)


Solution:13

There has been a lot of answers on general advices like data structure selection, access pattern, etc. Here I would like to add another code design pattern called software pipeline that makes use of active cache management.

The idea is borrow from other pipelining techniques, e.g. CPU instruction pipelining.

This type of pattern best applies to procedures that

  1. could be broken down to reasonable multiple sub-steps, S[1], S[2], S[3], ... whose execution time is roughly comparable with RAM access time (~60-70ns).
  2. takes a batch of input and do aforementioned multiple steps on them to get result.

Let's take a simple case where there is only one sub-procedure. Normally the code would like:

def proc(input):      return sub-step(input))  

To have better performance, you might want to pass multiple inputs to the function in a batch so you amortize function call overhead and also increases code cache locality.

def batch_proc(inputs):      results = []      for i in inputs:          // avoids code cache miss, but still suffer data(inputs) miss          results.append(sub-step(i))      return res  

However, as said earlier, if the execution of the step is roughly the same as RAM access time you can further improve the code to something like this:

def batch_pipelined_proc(inputs):      for i in range(0, len(inputs)-1):          prefetch(inputs[i+1])          # work on current item while [i+1] is flying back from RAM          results.append(sub-step(inputs[i-1]))        results.append(sub-step(inputs[-1]))  

The execution flow would look like:

  1. prefetch(1) ask CPU to prefetch input[1] into cache, where prefetch instruction takes P cycles itself and return, and in the background input[1] would arrive in cache after R cycles.
  2. works_on(0) cold miss on 0 and works on it, which takes M
  3. prefetch(2) issue another fetch
  4. works_on(1) if P + R <= M, then inputs[1] should be in the cache already before this step, thus avoid a data cache miss
  5. works_on(2) ...

There could be more steps involved, then you can design a multi-stage pipeline as long as the timing of the steps and memory access latency matches, you would suffer little code/data cache miss. However, this process needs to be tuned with many experiments to find out right grouping of steps and prefetch time. Due to its required effort, it sees more adoption in high performance data/packet stream processing. A good production code example could be found in DPDK QoS Enqueue pipeline design: http://dpdk.org/doc/guides/prog_guide/qos_framework.html Chapter 21.2.4.3. Enqueue Pipeline.

More information could be found:

https://software.intel.com/en-us/articles/memory-management-for-optimal-performance-on-intel-xeon-phi-coprocessor-alignment-and

http://infolab.stanford.edu/~ullman/dragon/w06/lectures/cs243-lec13-wei.pdf


Solution:14

Write your program to take a minimal size. That is why it is not always a good idea to use -O3 optimisations for GCC. It takes up a larger size. Often, -Os is just as good as -O2. It all depends on the processor used though. YMMV.

Work with small chunks of data at a time. That is why a less efficient sorting algorithms can run faster than quicksort if the data set is large. Find ways to break up your larger data sets into smaller ones. Others have suggested this.

In order to help you better exploit instruction temporal/spatial locality, you may want to study how your code gets converted in to assembly. For example:

for(i = 0; i < MAX; ++i)  for(i = MAX; i > 0; --i)  

The two loops produce different codes even though they are merely parsing through an array. In any case, your question is very architecture specific. So, your only way to tightly control cache use is by understanding how the hardware works and optimising your code for it.


Solution:15

Besides aligning your structure and fields, if your structure if heap allocated you may want to use allocators that support aligned allocations; like _aligned_malloc(sizeof(DATA), SYSTEM_CACHE_LINE_SIZE); otherwise you may have random false sharing; remember that in Windows, the default heap has a 16 bytes alignment.


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