Re: [PATCH] Increase core.packedGitLimit

From: Jeff King <peff@peff.net>
Date: Apr 20, 2017, 21:02 UTC
Message-ID: <20170420210254.f4ykyi46bso5uj3o@sigill.intra.peff.net>
In-Reply-To: <20170420204118.17856-1-dturner@twosigma.com>

On Thu, Apr 20, 2017 at 04:41:18PM -0400, David Turner wrote:

Show 18 quoted lines

> When core.packedGitLimit is exceeded, git will close packs.  If there
> is a repack operation going on in parallel with a fetch, the fetch
> might open a pack, and then be forced to close it due to
> packedGitLimit being hit.  The repack could then delete the pack
> out from under the fetch, causing the fetch to fail.
> 
> Increase core.packedGitLimit's default value to prevent
> this.
> 
> On current 64-bit x86_64 machines, 48 bits of address space are
> available.  It appears that 64-bit ARM machines have no standard
> amount of address space (that is, it varies by manufacturer), and IA64
> and POWER machines have the full 64 bits.  So 48 bits is the only
> limit that we can reasonably care about.  We reserve a few bits of the
> 48-bit address space for the kernel's use (this is not strictly
> necessary, but it's better to be safe), and use up to the remaining
> 45.  No git repository will be anywhere near this large any time soon,
> so this should prevent the failure.

Yep, I think this is a reasonable direction.

> ---
>  git-compat-util.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

This probably needs an update to the core.packedGitLimit section of Documentation/config.txt.

Show 10 quoted lines

> diff --git a/git-compat-util.h b/git-compat-util.h
> index 8a4a3f85e7..1c5de153a5 100644
> --- a/git-compat-util.h
> +++ b/git-compat-util.h
> @@ -616,7 +616,7 @@ extern int git_lstat(const char *, struct stat *);
>  #endif
>  
>  #define DEFAULT_PACKED_GIT_LIMIT \
> -	((1024L * 1024L) * (size_t)(sizeof(void*) >= 8 ? 8192 : 256))
> +	((1024L * 1024L) * (size_t)(sizeof(void*) >= 8 ? (32 * 1024L * 1024L) : 256))

I wondered if we would run afoul of integer sizes on 64-bit systems where "long" is still only 32-bits (i.e., Windows). But I think it's OK, because the values before we cast to size_t are in megabytes. So your 32*1024*1024 needs only 25 bits to store it. And then after we cast to size_t, everything is in 64-bit.

-Peff