Re: pack operation is thrashing my server

On Wed, 13 Aug 2008, Shawn O. Pearce wrote:

Nicolas Pitre [off-list ref] wrote:

quoted

You'll have memory usage issues whenever such objects are accessed, 
loose or not.  However, once those big objects are packed once, they can 
be repacked (or streamed over the net) without really "accessing" them.  
Packed object data is simply copied into a new pack in that case which 
is less of an issue on memory usage, irrespective of the original pack 
size.

And fortunately here we actually do stream the objects we have
chosen to reuse from the pack.  We don't allocate the entire thing
in memory.  Its probably the only place in all of Git where we can
handle a 16 GB (after compression) object on a machine with only
2 GB of memory and no swap.

Where little memory systems get into trouble with already packed
repositories is enumerating the objects to include in the pack.
This can still blow out their physical memory if the number of
objects to pack is high enough.  We need something like 160 bytes
of memory (my own memory is fuzzy on that estimate) per object.

I'm counting something like 104 bytes on a 64-bit machine for
struct object_entry.

Have 500k objects and its suddenly something quite real in terms
of memory usage.

Well, we are talking about 50MB which is not that bad.

However there is a point where we should be realistic and just admit 
that you need a sufficiently big machine if you have huge repositories 
to deal with.  Git should be fine serving pull requests with relatively 
little memory usage, but anything else such as the initial repack simply 
require enough RAM to be effective.


Nicolas