Re: [PATCH v3] packed_ref_cache: don't use mmap() for small files

Michael Haggerty [off-list ref] writes:

The change to using `read()` rather than `mmap()` for small
`packed-refs` feels like it should be an improvement, but it occurred to
me that the performance numbers quoted in ea68b0ce9f8 (hash-object:
don't use mmap() for small files, 2010-02-21) are not directly
applicable to the `packed-refs` file. As far as I understand, the file
mmapped in `index_fd()` is always read in full, whereas the main point
of mmapping the packed-refs file is to avoid having to read the whole
file at all in some situations. That being said, a 32 KiB file would
only be 8 pages (assuming a page size of 4 KiB), and by the time you've
read the header and binary-searched to find the desired record, you've
probably paged in most of the file anyway. Reading the whole file at
once, in order, is almost certainly cheaper.

Yup.  So unless your "small" is meaningfully large, we are likely to
be better off with read(2), but I suspect that this might not be
even measuable since we are only talking about "small" files.