Re: [PATCH] macOS: queue for munmap operations

From: Jeff King <peff@peff.net>
Date: Oct 21, 2025, 08:06 UTC
Message-ID: <20251021080625.GD259661@coredump.intra.peff.net>
In-Reply-To: <pull.1993.git.1760999702581.gitgitgadget@gmail.com>

On Mon, Oct 20, 2025 at 10:35:02PM +0000, Koji Nakamaru via GitGitGadget wrote:

Show 14 quoted lines

> From: Koji Nakamaru <koji.nakamaru@gree.net>
> 
> Executing many mmap/munmap calls alternately can cause a huge load on
> macOS. In order to reduce it, we should temporarily store munmap
> operations in a queue and process them all at once when the queue is
> filled. When the program terminates, we can discard any remaining munmap
> operations as corresponding mmaped regions are automatically reclaimed.
> 
> Add a queue for munmap operations to perform them all at once.
> 
> Here are some example timings. On the Linux kernel repository that
> requires about 1700 mmap/munmap calls:
> 
>   time git ls-tree -r -l --full-tree 211ddde > /dev/null

Why is it doing so many mmap calls? Do you have a ton of loose objects? We have to mmap loose objects individually (because they're all in separate files), but each pack only gets a single map (well, there's a window parameter, but it's 1GB on 64-bit systems, so you should get a handful of maps at most).

If you run "git gc", how does the resulting ls-tree perform? I have only 27 mmap() calls on my system.

I know that running "git gc" is relatively expensive, but it is also bringing other optimizations (like the fact that we don't have to open() and map each of those files in the first place!).

> On a private repository that requires about 943000 mmap/munmap calls:
> 
>   time git ls-tree -r -l --full-tree xxxxxxx > /dev/null

Ditto here. I'd be curious how well packed the repo is, and how it does after a repack. If it has a very large packfile, you might also try:

  git config core.packedGitWindowSize 4G

or similar (though for just an ls-tree, we should only be looking at tree objects, which in general I'd expect to be in a confined area of the packfile; so the 1GB window is probably plenty).

Show 20 quoted lines

> +int git_munmap(void *start, size_t length)
> +{
> +	static pthread_mutex_t mutex;
> +	static struct munmap_queue *queue;
> +	static int count;
> +	int i;
> +
> +	pthread_mutex_lock(&mutex);
> +	if (!queue)
> +		queue = xmalloc(COUNT_MAX * sizeof(struct munmap_queue));
> +	queue[count].start = start;
> +	queue[count].length = length;
> +	if (++count == COUNT_MAX) {
> +		for (i = 0; i < COUNT_MAX; i++)
> +			munmap(queue[i].start, queue[i].length);
> +		count = 0;
> +	}
> +	pthread_mutex_unlock(&mutex);
> +	return 0;
> +}

Does batching those unmaps actually make them faster? Or is it just that the commands you showed did not fill the queue, so we essentially just leaked all of those maps until the program exited?

If the latter, then I'd wonder:

  1. Does this increase memory pressure, since the OS has no idea we're
     not actually interested in those maps anymore? Some of them can be
     quite large, if the command is looking at blobs.

  2. How does it perform on a command that actually fills the queue? I
     guess something like "git log --raw" might do it (though if my
     guesses above are right, you'd need on the order of 64,000 loose
     trees).

-Peff