Re: Excessive mmap [was Git server eats all memory]

From: Ivan Kanis <expire-by-2010-08-14@kanis.fr>
Date: Aug 9, 2010, 16:34 UTC
Message-ID: <westyn3n3sa.fsf@kanis.fr>
In-Reply-To: <AANLkTi=6JcwLuyNWq9oYzE_E+7DSn-sEvR-X3AHvXM6U@mail.gmail.com>

Dmitry Potapov <dpotapov@gmail.com> wrote:

> Though Git uses MAP_PRIVATE with mmap, this only marks mapped pages as
> copy-on-write. Because cloning does not change the pack file, all those
> pages should be shared.

I reran the test today. One client is receiving while the other one is compressing. I have to interrupt both client because the server is becoming unusable. I am sure you are right about sharing the pages but I can't test it.

> On 64-bit architecture, you have plenty virtual space, and mapping
> a file to memory should not take much physical memory (only space
> needed for system tables).

What I can tell from the mmap man page is that it should map memory to a file. I assume it shouldn't take up physical memory. However I am seeing physical memory being consumed. It might be a feature of the kernel. Is there a way to turn it off?

> You can reduce core.packedGitWindowSize in the Git configuration to
> see if it helps, but I doubt that it will have any noticeable effect.

It was worth a shot, it didn't help...

Looking some more into it today the bulk of the memory allocation happens in write_pack_file in the following loop.

for (; i < nr_objects; i++) {
    if (!write_one(f, objects + i, &offset))
        break;
    display_progress(progress_state, written);
}

This eventually calls write_object, here I am wondering if the unuse_pack function is doing its job. As far as I can tell it writes a null in memory, that I think is not enough to reclaim memory.

I also looked at the use_pack function where the mmap is happening. Would it be worth refactoring this function so that it uses an index withing a file instead of mmap?

Unless I hear of a better idea I'll be trying that tomorrow...

Take care,

-- 
Ivan Kanis

I can stand brute force, but brute reason is quite unbearable.  There
is something unfair about its use. It is hitting below the intellect.
    -- Oscar Wilde