Re: [PATCH/RFC] Allow writing loose objects that are corrupted in a pack file

[Linus Torvalds <torvalds@linux-foundation.org>]

On Wed, 7 Jan 2009, R. Tyler Ballance wrote:
> 
> I was only mentioning OS X with regards to the Samba/NFS red herring,
> the rest of our operations are on 64-bit Linux machines.

Ahh, ok. Good. 

> > I could easily see that if you have a virtual memory size limit of 1.5GB, 
> > and the pack window size is 1GB, we might have trouble. Because we could 
> > only keep one such pack window in memory at a time.
> 
> The DEFAULT_PACKED_GIT_WINDOW_SIZE in our local builds is 256M, FWIW

Interesting. So you already had to lower it. However, now that you mention 
it, and now that I search for your emails about it on the mailing list (I 
don't normally read the mailing list except very occasionally), I see your 
patch that does

	#define DYNAMIC_WINDOW_SIZE_PERCENTAGE 0.85
	...
	packed_git_window_size = (unsigned int)(as->rlim_cur * DYNAMIC_WINDOW_SIZE_PERCENTAGE);

which is actually very bad.

It's bad for several reasons:

 - 85% of the virtual address space is actually pessimal.

   You need space for AT LEAST two full-sized windows, so you need less 
   than 50%.

 - the way that variable is used, it _has_ to be a multiple of the page 
   size. In fact, it needs to be a multiple of _twice_ the page size. So 
   just doing a random fraction of the rlimit is not correct.

Setting it in the .gitconfig does it right, though.

> > If so, then adding a
> > 
> > 	[core]
> > 		packedgitwindowsize = 64M
> > 
> > might make a difference. It would certainly be very interesting to hear if 
> > there's any impact.
> 
> I can try this still if you'd like, but it doesn't seem like that'd be
> the issue since we're already lowering the window size system-wide

Please do try, at least if your local git changes still match that patch I 
found, because that patch generates problems.

		Linus