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Re: Reduce pack-objects memory footprint?

On Wed, Feb 28, 2018 at 4:27 PM, Duy Nguyen <pclouds@xxxxxxxxx> wrote:
> linux-2.6.git current has 6483999 objects. "git gc" on my poor laptop
> consumes 1.7G out of 4G RAM, pushing lots of data to swap and making
> all apps nearly unusuable (granted the problem is partly Linux I/O
> scheduler too). So I wonder if we can reduce pack-objects memory
> footprint a bit.

Next low hanging fruit item:

struct revindex_entry {
        off_t offset;
        unsigned int nr;

We need on entry per object, so 6.5M objects * 16 bytes = 104 MB. If
we break this struct apart and store two arrays of offset and nr in
struct packed_git, we save 4 bytes per struct, 26 MB total.

It's getting low but every megabyte counts for me, and it does not
look like breaking this struct will make horrible code (we recreate
the struct at find_pack_revindex()) so I'm going to do this too unless
someone objects. There will be slight performance regression due to
cache effects, but hopefully it's ok.