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330 lines
15 KiB
Markdown
330 lines
15 KiB
Markdown
# Fast directory listing
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In order to find untracked files in a git repository, [gitstatusd](../README.md) needs to list the
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contents of every directory. gitstatusd does it 27% faster than a reasonable implementation that a
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seasoned C/C++ practitioner might write. This document explains the optimizations that went into it.
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As directory listing is a common operation, many other projects can benefit from applying these
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optimizations.
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## v1
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Given a path to a directory, `ListDir()` must produce the list of files in that directory. Moreover,
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the list must be sorted lexicographically to enable fast comparison with Git index.
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The following C++ implementation gets the job done. For simplicity, it returns an empty list on
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error.
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```c++
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vector<string> ListDir(const char* dirname) {
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vector<string> entries;
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if (DIR* dir = opendir(dirname)) {
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while (struct dirent* ent = (errno = 0, readdir(dir))) {
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if (!Dots(ent->d_name)) entries.push_back(ent->d_name);
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}
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if (errno) entries.clear();
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sort(entries.begin(), entries.end());
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closedir(dir);
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}
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return entries;
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}
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```
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Every directory has entries `"."` and `".."`, which we aren't interested in. We filter them out with
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a helper function `Dots()`.
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```c++
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bool Dots(const char* s) { return s[0] == '.' && (!s[1] || (s[1] == '.' && !s[2])); }
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```
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To check how fast `ListDir()` performs, we can run it many times on a typical directory. One million
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runs on a directory with 32 files with 16-character names takes 12.7 seconds.
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## v2
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Experienced C++ practitioners will scoff at our implementation of `ListDir()`. If it's meant to be
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efficient, returning `vector<string>` is an unaffordable convenience. To avoid heap allocations we
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can use a simple arena that will allow us to reuse memory between different `ListDir()` calls.
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(Changed and added lines are marked with comments.)
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```c++
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void ListDir(const char* dirname, string& arena, vector<char*>& entries) { // +
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entries.clear(); // +
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if (DIR* dir = opendir(dirname)) {
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arena.clear(); // +
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while (struct dirent* ent = (errno = 0, readdir(dir))) {
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if (!Dots(ent->d_name)) {
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entries.push_back(reinterpret_cast<char*>(arena.size())); // +
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arena.append(ent->d_name, strlen(ent->d_name) + 1); // +
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}
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}
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if (errno) entries.clear();
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for (char*& p : entries) p = &arena[reinterpret_cast<size_t>(p)]; // +
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sort(entries.begin(), entries.end(), // +
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[](const char* a, const char* b) { return strcmp(a, b) < 0; }); // +
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closedir(dir);
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}
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}
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```
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To make performance comparison easier, we can normalize them relative to the baseline. v1 will get
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performance score of 100. A twice-as-fast alternative will be 200.
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| version | optimization | score |
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|---------|----------------------------|----------:|
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| v1 | baseline | 100.0 |
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| **v2** | **avoid heap allocations** | **112.7** |
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Avoiding heap allocations makes `ListDir()` 12.7% faster. Not bad. As an added bonus, those casts
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will fend off the occasional frontend developer who accidentally wanders into the codebase.
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## v3
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`opendir()` is an expensive call whose performance is linear in the number of subdirectories in the
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path because it needs to perform a lookup for every one of them. We can replace it with `openat()`,
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which takes a file descriptor to the parent directory and a name of the subdirectory. Just a single
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lookup, less CPU time. This optimization assumes that callers already have a descriptor to the
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parent directory, which is indeed the case for gitstatusd, and is often the case in other
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applications that traverse filesystem.
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```c++
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void ListDir(int parent_fd, const char* dirname, string& arena, vector<char*>& entries) { // +
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entries.clear();
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int dir_fd = openat(parent_fd, dirname, O_NOATIME | O_RDONLY | O_DIRECTORY | O_CLOEXEC); // +
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if (dir_fd < 0) return; // +
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if (DIR* dir = fdopendir(dir_fd)) {
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arena.clear();
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while (struct dirent* ent = (errno = 0, readdir(dir))) {
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if (!Dots(ent->d_name)) {
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entries.push_back(reinterpret_cast<char*>(arena.size()));
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arena.append(ent->d_name, strlen(ent->d_name) + 1);
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}
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}
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if (errno) entries.clear();
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for (char*& p : entries) p = &arena[reinterpret_cast<size_t>(p)];
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sort(entries.begin(), entries.end(),
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[](const char* a, const char* b) { return strcmp(a, b) < 0; });
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closedir(dir);
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} else { // +
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close(dir_fd); // +
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} // +
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}
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```
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This is worth about 3.5% in speed.
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| version | optimization | score |
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|---------|--------------------------------------|----------:|
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| v1 | baseline | 100.0 |
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| v2 | avoid heap allocations | 112.7 |
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| **v3** | **open directories with `openat()`** | **116.2** |
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## v4
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Copying file names to the arena isn't free but it doesn't seem like we can avoid it. Poking around
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we can see that the POSIX API we are using is implemented on Linux on top of `getdents64` system
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call. Its documentation isn't very encouraging:
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```text
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These are not the interfaces you are interested in. Look at
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readdir(3) for the POSIX-conforming C library interface. This page
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documents the bare kernel system call interfaces.
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Note: There are no glibc wrappers for these system calls.
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```
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Hmm... The API looks like something we can take advantage of, so let's try it anyway.
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First, we'll need a simple `Arena` class that can allocate 8KB blocks of memory.
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```c++
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class Arena {
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public:
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enum { kBlockSize = 8 << 10 };
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char* Alloc() {
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if (cur_ == blocks_.size()) blocks_.emplace_back(kBlockSize, 0);
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return blocks_[cur_++].data();
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}
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void Clear() { cur_ = 0; }
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private:
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size_t cur_ = 0;
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vector<string> blocks_;
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};
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```
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Next, we need to define `struct dirent64_t` ourselves because there is no wrapper for the system
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call we are about to use.
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```c++
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struct dirent64_t {
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ino64_t d_ino;
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off64_t d_off;
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unsigned short d_reclen;
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unsigned char d_type;
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char d_name[];
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};
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```
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Finally we can get to the implementation of `ListDir()`.
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```c++
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void ListDir(int parent_fd, Arena& arena, vector<char*>& entries) { // +
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entries.clear();
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int dir_fd = openat(parent_fd, dirname, O_NOATIME | O_RDONLY | O_DIRECTORY | O_CLOEXEC);
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if (dir_fd < 0) return;
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arena.Clear(); // +
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while (true) { // +
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char* buf = arena.Alloc(); // +
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int n = syscall(SYS_getdents64, dir_fd, buf, Arena::kBlockSize); // +
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if (n <= 0) { // +
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if (n) entries.clear(); // +
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break; // +
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} // +
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for (int pos = 0; pos < n;) { // +
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auto* ent = reinterpret_cast<dirent64_t*>(buf + pos); // +
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if (!Dots(ent->d_name)) entries.push_back(ent->d_name); // +
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pos += ent->d_reclen; // +
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} // +
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} // +
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sort(entries.begin(), entries.end(),
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[](const char* a, const char* b) { return strcmp(a, b) < 0; });
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close(dir_fd);
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}
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```
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How are we doing with this one?
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| version | optimization | score |
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|---------|----------------------------------|----------:|
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| v1 | baseline | 100.0 |
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| v2 | avoid heap allocations | 112.7 |
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| v3 | open directories with `openat()` | 116.2 |
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| **v4** | **call `getdents64()` directly** | **137.8** |
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Solid 20% speedup. Worth the trouble. Unfortunately, we now have just one `reinterpret_cast` instead
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of two, and it's not nearly as scary-looking. Hopefully with the next iteration we can get back some
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of that evil vibe of low-level code.
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As a bonus, every element in `entries` has `d_type` at offset -1. This can be useful to the callers
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that need to distinguish between regular files and directories (gitstatusd, in fact, needs this).
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Note how `ListDir()` implements this feature at zero cost, as a lucky accident of `dirent64_t`
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memory layout.
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## v5
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The CPU profile of `ListDir()` reveals that almost all userspace CPU time is spent in `strcmp()`.
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Digging into the source code of `std::sort()` we can see that it uses Insertion Sort for short
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collections. Our 32-element vector falls under the threshold. Insertion Sort makes `O(N^2)`
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comparisons, hence a lot of CPU time in `strcmp()`. Switching to `qsort()` or
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[Timsort](https://en.wikipedia.org/wiki/Timsort) is of no use as all good sorting algorithms fall
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back to Insertion Sort.
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If we cannot make fewer comparisons, perhaps we can make each of them faster? `strcmp()` compares
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characters one at a time. It cannot read ahead as it can be illegal to touch memory past the first
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null byte. But _we_ know that it's safe to read a few extra bytes past the end of `d_name` for every
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entry except the last in the buffer. And since we own the buffer, we can overallocate it so that
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reading past the end of the last entry is also safe.
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Combining these ideas with the fact that file names on Linux are at most 255 bytes long, we can
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invoke `getdents64()` like this:
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```c++
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int n = syscall(SYS_getdents64, dir_fd, buf, Arena::kBlockSize - 256);
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```
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And then compare entries like this:
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```c++
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[](const char* a, const char* b) { return memcmp(a, b, 255) < 0; }
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```
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This version doesn't give any speedup compared to the previous but it opens an avenue for another
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optimization. The pointers we pass to `memcmp()` aren't aligned. To be more specific, their
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numerical values are `N * 8 + 3` for some `N`. When given such a pointer, `memcmp()` will check the
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first 5 bytes one by one, and only then switch to comparing 8 bytes at a time. If we can handle the
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first 5 bytes ourselves, we can pass aligned memory to `memcmp()` and take full advantage of its
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vectorized loop.
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Here's the implementation:
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```c++
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uint64_t Read64(const void* p) { // +
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uint64_t x; // +
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memcpy(&x, p, sizeof(x)); // +
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return x; // +
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} // +
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void ByteSwap64(void* p) { // +
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uint64_t x = __builtin_bswap64(Read64(p)); // +
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memcpy(p, &x, sizeof(x)); // +
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} // +
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void ListDir(int parent_fd, Arena& arena, vector<char*>& entries) {
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entries.clear();
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int dir_fd = openat(parent_fd, dirname, O_NOATIME | O_RDONLY | O_DIRECTORY | O_CLOEXEC);
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if (dir_fd < 0) return;
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arena.Clear();
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while (true) {
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char* buf = arena.Alloc();
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int n = syscall(SYS_getdents64, dir_fd, buf, Arena::kBlockSize - 256); // +
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if (n <= 0) {
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if (n) entries.clear();
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break;
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}
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for (int pos = 0; pos < n;) {
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auto* ent = reinterpret_cast<dirent64_t*>(buf + pos);
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if (!Dots(ent->d_name)) {
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ByteSwap64(ent->d_name); // +
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entries.push_back(ent->d_name);
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}
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pos += ent->d_reclen;
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}
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}
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sort(entries.begin(), entries.end(), [](const char* a, const char* b) {
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uint64_t x = Read64(a); // +
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uint64_t y = Read64(b); // +
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return x < y || (x == y && a != b && memcmp(a + 5, b + 5, 256) < 0); // +
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});
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for (char* p : entries) ByteSwap64(p); // +
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close(dir_fd);
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}
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```
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This is for Little Endian architecture. Big Endian doesn't need `ByteSwap64()`, so it'll be a bit
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faster.
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| version | optimization | score |
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|---------|----------------------------------|----------:|
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| v1 | baseline | 100.0 |
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| v2 | avoid heap allocations | 112.7 |
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| v3 | open directories with `openat()` | 116.2 |
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| v4 | call `getdents64()` directly | 137.8 |
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| **v5** | **hand-optimize `strcmp()`** | **143.3** |
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Fast and respectably arcane.
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## Conclusion
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Through a series of incremental improvements we've sped up directory listing by 43.3% compared to a
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naive implementation (v1) and 27.2% compared to a reasonable implementation that a seasoned C/C++
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practitioner might write (v2).
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However, these numbers are based on an artificial benchmark while the real judge is always the real
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code. Our goal was to speed up gitstatusd. Benchmark was just a tool. Thankfully, the different
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versions of `ListDir()` have the same comparative performance within gitstatusd as in the benchmark.
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In truth, the directory chosen for the benchmark wasn't arbitrary. It was picked by sampling
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gitstatusd when it runs on [chromium](https://github.com/chromium/chromium) git repository.
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The final version of `ListDir()` spends 97% of its CPU time in the kernel. If we assume that it
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makes the minimum possible number of system calls and these calls are optimal (true to the best
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of my knowledge), it puts the upper bound on possible future performance improvements at just 3%.
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There is almost nothing left in `ListDir()` to optimize.
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(The CPU profile was created with [gperftools](https://github.com/gperftools/gperftools) and
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rendered with [pprof](https://github.com/google/pprof)).
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