Re: [PATCH] blame.c: don't drop origin blobs as eagerly

From: Junio C Hamano <gitster@pobox.com>
Date: Apr 3, 2019, 07:45 UTC
Message-ID: <xmqqv9zvsfay.fsf@gitster-ct.c.googlers.com>
In-Reply-To: <20190402115625.21427-1-dak@gnu.org>

David Kastrup <dak@gnu.org> writes:

Show 8 quoted lines

> When a parent blob already has chunks queued up for blaming, dropping
> the blob at the end of one blame step will cause it to get reloaded
> right away, doubling the amount of I/O and unpacking when processing a
> linear history.
>
> Keeping such parent blobs in memory seems like a reasonable optimization
> that should incur additional memory pressure mostly when processing the
> merges from old branches.

Thanks for finding an age-old one that dates back to 7c3c7962 ("blame: drop blob data after passing blame to the parent", 2007-12-11).

Interestingly, the said commit claims:

    When passing blame from a parent to its parent (i.e. the
    grandparent), the blob data for the parent may need to be read
    again, but it should be relatively cheap, thanks to delta-base
    cache.
            
but perhaps you found a case where the delta-base cache is not all
that effective in the benchmark?

Will queue.  Thanks.

Show 20 quoted lines

>
> Signed-off-by: David Kastrup <dak@gnu.org>
> ---
>  blame.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/blame.c b/blame.c
> index 5c07dec190..c11c516921 100644
> --- a/blame.c
> +++ b/blame.c
> @@ -1562,7 +1562,8 @@ static void pass_blame(struct blame_scoreboard *sb, struct blame_origin *origin,
>  	}
>  	for (i = 0; i < num_sg; i++) {
>  		if (sg_origin[i]) {
> -			drop_origin_blob(sg_origin[i]);
> +			if (!sg_origin[i]->suspects)
> +				drop_origin_blob(sg_origin[i]);
>  			blame_origin_decref(sg_origin[i]);
>  		}
>  	}