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Re: Reduce pack-objects memory footprint?

On Fri, Mar 02, 2018 at 05:18:45PM +0700, Duy Nguyen wrote:

> 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.

Maybe you will prove me wrong, but I don't think splitting them is going
to work. The point of the revindex_entry is that we sort the (offset,nr)
tuple as a unit.

Or are you planning to sort it, and then copy the result into two
separate arrays? I think that would work, but it sounds kind of nasty
(arcane code, and extra CPU work for systems that don't care about the