From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from pb-smtp21.pobox.com (pb-smtp21.pobox.com [173.228.157.53]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D3C204A11 for ; Thu, 16 May 2024 22:43:16 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=173.228.157.53 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1715899398; cv=none; b=C4pvYXcO7XvMH0S+5oJ9iX1pit0WyYS8gDhJWovaBRW9+9QXx4/5ICRV5alE+u1cRLY0+UaMR+EL9mIRm5YqQ8ujC5hQ6bDE8EN/SzXJXp5ycxh27Uv+sPY9wzDWhv0ODerVY7B47tuDYmAMypRQjxPe6EBmlTkyuffFlKnVxqE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1715899398; c=relaxed/simple; bh=llOiJWZJ0N/U7cHlgpX5eUIYO4ab1KmIO8wdTOyXSZ8=; h=From:To:Cc:Subject:In-Reply-To:References:Date:Message-ID: MIME-Version:Content-Type; b=NSRKzAqVFXyMhlbI3+eTZs3QN/qFOoY5OIgxXh+wWeLbeO8y5RhUKCn+ZmGUVRV0JihwEQRL6KmzM0xXbNAZX2MCaisv4Q8lZ01wIi0SFBFHOyx8pq6nH+jv5BoAr14CEOdYwr6UZPL1MjV+Hs0vWDmSszjQcbCIeXfSnpLureE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=pobox.com; spf=pass smtp.mailfrom=pobox.com; dkim=pass (1024-bit key) header.d=pobox.com header.i=@pobox.com header.b=PgLSxrkm; arc=none smtp.client-ip=173.228.157.53 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=pobox.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=pobox.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=pobox.com header.i=@pobox.com header.b="PgLSxrkm" Received: from pb-smtp21.pobox.com (unknown [127.0.0.1]) by pb-smtp21.pobox.com (Postfix) with ESMTP id 4887429516; Thu, 16 May 2024 18:43:16 -0400 (EDT) (envelope-from junio@pobox.com) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed; d=pobox.com; h=from:to:cc :subject:in-reply-to:references:date:message-id:mime-version :content-type; s=sasl; bh=llOiJWZJ0N/U7cHlgpX5eUIYO4ab1KmIO8wdTO yXSZ8=; b=PgLSxrkm9nzabF6Ji+ufWOi5uocUrY9b8ncRFFx6N14UWNBTYOBnYf wM7TJnAZG9bYUluGpTi8R84CeFLq3jGd2GTCxRIQblWkLDizf7uehM+f7Tda5ItW t/TjNvto7JyJKzzIwmLgdYOp0C4V8WBpxVKsDiXekvQqNrtMkfxL4= Received: from pb-smtp21.sea.icgroup.com (unknown [127.0.0.1]) by pb-smtp21.pobox.com (Postfix) with ESMTP id 4051B29515; Thu, 16 May 2024 18:43:16 -0400 (EDT) (envelope-from junio@pobox.com) Received: from pobox.com (unknown [34.125.153.120]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by pb-smtp21.pobox.com (Postfix) with ESMTPSA id CBD2A29514; Thu, 16 May 2024 18:43:12 -0400 (EDT) (envelope-from junio@pobox.com) From: Junio C Hamano To: Jeff King Cc: Taylor Blau , git@vger.kernel.org, Elijah Newren Subject: Re: [PATCH v2 09/23] pseudo-merge: implement support for selecting pseudo-merge commits In-Reply-To: <20240516080704.GA85197@coredump.intra.peff.net> (Jeff King's message of "Thu, 16 May 2024 04:07:04 -0400") References: <86a1e4b8b9be99563836d1539fbf2ed4c4a6920d.1714422410.git.me@ttaylorr.com> <20240513190340.GC2675586@coredump.intra.peff.net> <20240516080704.GA85197@coredump.intra.peff.net> Date: Thu, 16 May 2024 15:43:11 -0700 Message-ID: User-Agent: Gnus/5.13 (Gnus v5.13) Precedence: bulk X-Mailing-List: git@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain X-Pobox-Relay-ID: AD50E59C-13D5-11EF-81F0-A19503B9AAD1-77302942!pb-smtp21.pobox.com Jeff King writes: > motivation. Something like (assuming we're following a section on > bitmaps in the "advanced packing" page, something I recognize also > does not yet exist): > > Reachability bitmaps are most efficient when we have on-disk > stored bitmaps for one or more of the starting points of a > traversal. For this reason, Git prefers storing bitmaps for > commits at the tips of refs, because traversals tend to start with > those points. > > But if you have a large number of refs, it's not feasible to store > a bitmap for _every_ ref tip. It takes up space, and just OR-ing > all of those bitmaps together is expensive. > > One way we can deal with that is to create bitmaps that represent > _groups_ of refs. When a traversal asks about the entire group, > then we can use this single bitmap instead of considering each ref > individually. Because these bitmaps represent the set of objects > which would be reachable in a hypothetical merge of all of the > commits, we call them pseudo-merge bitmaps. Nicely put. I wish there were something like the above in the patches when I read these patches for the first time. The concept of "pseudo-merge" was the first hump in the road to understanding. Eventually I figured it out, but a simple write-up like the above would have helped readers a lot. Thanks.