linux-kernel - Re: [PATCH v4 00/14] Add kdbus implementation

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Date:	Wed, 18 Mar 2015 14:54:12 +0100
From:	David Herrmann <dh.herrmann@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Arnd Bergmann <arnd@...db.de>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Tom Gundersen <teg@...m.no>, Jiri Kosina <jkosina@...e.cz>,
	Linux API <linux-api@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Daniel Mack <daniel@...que.org>,
	Djalal Harouni <tixxdz@...ndz.org>
Subject: Re: [PATCH v4 00/14] Add kdbus implementation

Hi

On Tue, Mar 17, 2015 at 8:24 PM, Andy Lutomirski <luto@...capital.net> wrote:
[...]
> Can anyone comment on how fast it actually is.  I'm curious
> about:
>
>  - The time it takes to do the ioctl to send a message
>
>  - The time it takes to receive a message (poll + whatever ioctls)

I'm not sure I can gather useful absolute data here. This highly
depends on how you call it, how often you call it, what payloads you
pass, what machine you're on.. you know all that.

So here's a flamegraph for you (which I use for comparisons to UDS):
  http://people.freedesktop.org/~dvdhrm/kdbus_8kb.svg

This program sends unicast messages on kdbus and UDS, exactly the same
number of times with the same 8kb payload. No parsing, no marshaling
is done, just plain message passing. The interesting spikes are
sys_read(), sys_write() and the 3 kdbus sys_ioctl(). Everything else
should be ignored.

sys_read() and sys_ioctl(KDBUS_CMD_RECV) are about the same. But note
that we don't copy any payload in RECV, so it scales O(1) compared to
message-size.

sys_write() is about 3x faster than sys_ioctl(KDBUS_CMD_WRITE).

I see lots of room for improvement in both RECV and SEND. Caching the
namespaces on a connection, would get rid of
kdbus_queue_entry_install() in RECV, thus speeding it up by ~30%. In
SEND, we could merge the kvec and iovec copying, to avoid calling
shmem_begin_write() twice. We should also stop allocating management
structures that are not used (like for metadata, if no metadata is
transmitted). We should use stack-space for small ioctl objects,
instead of memdup_user(). And so on.. Oh, and locking can be reduced.
We haven't even looked at rcu, yet (though that's mostly interesting
for policy and broadcasts, not unicasts).

>  - The time it takes to transfer a memfd (I don't care about how long
> it takes to create or map a memfd -- that's exactly the same between
> kdbus and any other memfds user, I imagine)

The time to transmit a memfd is the same as to transmit a 64-byte
payload. Ok, you also get to install the fd into the fd-table, but
that's true regardless of the transport.
Here's a graph for 64byte transfers (otherwise, same as above):
  http://people.freedesktop.org/~dvdhrm/kdbus_64b.svg

>  - The time it takes to connect

No idea, never measured it. Why is it of interest?

> I'm also interested in whether the current design is actually amenable
> to good performance.  I'm told that it is, but ISTM there's a lot of
> heavyweight stuff going on with each send operation that will be hard
> to remove.

I disagree. What heavyweight stuff is going on?

Thanks
David
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