Re: [PATCH 16/20] builtin/gc.c: guess the size of the revindex

On 1/8/2021 1:17 PM, Taylor Blau wrote:

'estimate_repack_memory()' takes into account the amount of memory
required to load the reverse index in memory by multiplying the assumed
number of objects by the size of the 'revindex_entry' struct.

Prepare for hiding the definition of 'struct revindex_entry' by removing
a 'sizeof()' of that type from outside of pack-revindex.c. Instead,
guess that one off_t and one uint32_t are required per object. Strictly
speaking, this is a worse guess than asking for 'sizeof(struct
revindex_entry)' directly, since the true size of this struct is 16
bytes with padding on the end of the struct in order to align the offset
field.

This is so far the only not-completely-obvious change.
 

But, this is an approximation anyway, and it does remove a use of the
'struct revindex_entry' from outside of pack-revindex internals.

And this might be enough justification for it, but...

-	heap += sizeof(struct revindex_entry) * nr_objects;
+	heap += (sizeof(off_t) + sizeof(uint32_t)) * nr_objects;

...outside of the estimation change, will this need another change
when the rev-index is mmap'd? Should this instead be an API call,
such as estimate_rev_index_memory(nr_objects)? That would
centralize the estimate to be next to the code that currently
interacts with 'struct revindex_entry' and will later interact with
the mmap region.

Thanks,
-Stolee