Re: [PATCH v2 1/2] sha1_file: introduce close_one_pack() to close packs on fd pressure

[Brandon Casey <drafnel@gmail.com>]

On Thu, Aug 1, 2013 at 12:16 PM, Brandon Casey <drafnel@gmail.com> wrote:
> On Thu, Aug 1, 2013 at 11:39 AM, Junio C Hamano <gitster@pobox.com> wrote:
>> Brandon Casey <drafnel@gmail.com> writes:
>>
>>> I've been looking closer at uses of p->windows everywhere, and it
>>> seems that we always open_packed_git() before we try to create new
>>> windows.  There doesn't seem to be any reason that we can't continue
>>> to use the existing open windows even after closing the pack file.
>>> ...
>>> If we don't need to close_pack_windows(), find_lru_pack() doesn't
>>> strictly need to reject packs that have windows in use.
>>
>> That makes me feel somewhat uneasy.  Yes, you can open/mmap/close
>> and hold onto the contents of a file still mapped in-core, and it
>> may not count as "open filedescriptor", but do OSes allow infinite
>> such mmapped regions to us?  We do keep track of number of open
>> windows, but is there a way for us to learn how close we are to the
>> limit?
>
> Not that I know of, but xmmap() does already try to unmap existing
> windows when mmap() fails, and then retries the mmap.  It calls
> release_pack_memory() which calls unuse_one_window().  mmap returns
> ENOMEM when either there is no available memory or if the limit of
> mmap mappings has been exceeded.
>
> So, I think we'll be ok.  It's the same situation we'd be in if there
> were many large packs (but fewer than pack_max_fds) and a small
> packedGitWindowSize, requiring many mmap windows.  We'd try to map an
> additional segment, fail, release some unused segments, and retry.

Also, it's the same situation we'd be in if there were many small
packs that were smaller than packedGitWindowSize.  We'd mmap the
entire pack file into memory and then close the file descriptor,
allowing us to have many more pack files mapped into memory than
pack_max_fds would allow us to have open.  With enough small packs,
we'd eventually reach the mmap limit and xmmap would try to release
some mappings.

-Brandon