Powerlevel10k

For the highest performance, bypass the POSIX readdir() interface and call the Linux getdents64 system call directly. This requires defining a custom dirent64_t structure and managing an Arena for memory allocation. This approach yields a ~37.8% speedup over the baseline (v1).

struct dirent64_t {
  ino64_t d_ino;
  off64_t d_off;
  unsigned short d_reclen;
  unsigned char d_type;
  char d_name[];
};

void ListDir(int parent_fd, Arena& arena, vector<char*>& entries) {
  entries.clear();
  int dir_fd = openat(parent_fd, dirname, O_NOATIME | O_RDONLY | O_DIRECTORY | O_CLOEXEC);
  if (dir_fd < 0) return;
  arena.Clear();
  while (true) {
    char* buf = arena.Alloc();
    int n = syscall(SYS_getdents64, dir_fd, buf, Arena::kBlockSize);
    if (n <= 0) {
      if (n) entries.clear();
      break;
    }
    for (int pos = 0; pos < n;) {
      auto* ent = reinterpret_cast<dirent64_t*>(buf + pos);
      if (!Dots(ent->d_name)) entries.push_back(ent->d_name);
      pos += ent->d_reclen;
    }
  }
  sort(entries.begin(), entries.end(),
       [](const char* a, const char* b) { return strcmp(a, b) < 0; });
  close(dir_fd);
}

The ListDir() function in gitstatus has been optimized through several iterations to minimize userspace CPU time. The most significant bottleneck identified was strcmp() during sorting, which performs $O(N^2)$ comparisons for small collections due to Insertion Sort.

To achieve maximum performance (v5), the implementation uses the following strategies:

1. Direct System Calls: Using getdents64() directly instead of standard library wrappers.
2. Memory Alignment & Vectorization: To avoid the overhead of memcmp() checking unaligned pointers byte-by-byte, the implementation uses ByteSwap64 to align the first 8 bytes of the filename. This allows memcmp() to immediately utilize vectorized loops.
3. Custom Sorting Logic: A custom comparator is used with std::sort that reads the first 64 bits of the filename as a uint64_t for rapid comparison, falling back to memcmp for the remaining bytes if the prefixes are equal.

Performance Evolution

Final optimized versions spend approximately 97% of CPU time in the kernel, leaving minimal room for further userspace optimization.

// Example of the v5 optimized sorting comparator logic
sort(entries.begin(), entries.end(), [](const char* a, const char* b) {
  uint64_t x = Read64(a);
  uint64_t y = Read64(b);
  return x < y || (x == y && a != b && memcmp(a + 5, b + 5, 256) < 0);
});

Instant prompt speeds up Zsh startup by rendering the prompt before the rest of the initialization is complete. You can enable it via p10k configure or by manually adding the preamble to the very top of your ~/.zshrc.

Important Rules:

1. Copy the preamble code verbatim.
2. Any initialization code that requires console input (e.g., password prompts, [y/n] confirmations) must be placed above the instant prompt block.
3. Code that only prints to the console (without reading input) can go below, but output might appear uncolored.

Configuration Options (POWERLEVEL9K_INSTANT_PROMPT):

• off: Completely disables instant prompt.
• quiet: Silences warnings about console output during initialization (recommended if you have non-interactive print statements).
• verbose (default): Prints a warning if console output is detected during initialization.

# Enable Powerlevel10k instant prompt. Should stay close to the top of ~/.zshrc.
# Initialization code that may require console input (password prompts, [y/n]
# confirmations, etc.) must go above this block; everything else may go below.
if [[ -r "${XDG_CACHE_HOME:-$HOME/.cache}/p10k-instant-prompt-${(%):-%n}.zsh" ]]; then
  source "${XDG_CACHE_HOME:-$HOME/.cache}/p10k-instant-prompt-${(%):-%n}.zsh"
fi

gitstatusd is designed for high-performance git status reporting, significantly outperforming standard tools like libgit2 or git status in many scenarios. Its speed is derived from several key architectural choices:

• Efficient System Calls: Instead of using lstat() (which requires path lookups for every subdirectory), it uses fstatat() and openat(), which operate relative to a parent directory file descriptor. This reduces CPU time in the kernel.
• Parallelism: The diffing algorithm is designed for multi-threading, allowing it to utilize multiple CPU cores to scan the index and workdir in parallel with near-perfect scaling.
• Untracked Cache: It remembers the last modification time of directories. On subsequent scans, if a directory's modification time hasn't changed, gitstatusd skips re-scanning it for untracked files.
• Optimized Data Structures: It uses performance-conscious coding styles and efficient algorithms to reduce CPU time in userspace.
• Direct System Calls: On Linux, it uses the getdents64 system call directly to bypass glibc wrappers, improving directory listing speed.