How Debian managed the systemd transition [LWN.net]

How Debian managed the systemd transition

Posted Sep 17, 2015 5:52 UTC (Thu) by luto (subscriber, #39314)
In reply to: How Debian managed the systemd transition by josh
Parent article: How Debian managed the systemd transition

I'm not really convinced by this context switch thing. For a messaging system, users are likely to care about latency and about throughput. Certainly, to send a single message via a central daemon, two context switches are required, whereas sending a message via kdbus or any other direct-through-the-kernel system only needs one context switch.

But context switches should be decently under 2 µs on a modern system. (The atrocious performance of libgdbus + dbus-daemon has *nothing* do with with the extra context switch.) With some optimization, which certainly could be done, I bet we can significantly improve context switches performance.

In any event, for applications that care about throughput, the extra context switch is a red herring. Under load, a good central daemon will process many messages per time slice, so the throughput bottleneck is much more likely to be message routing and such rather than context switches. Under that type of load, having a central daemon shouldn't by much slower than doing everything in the kernel. Kdbus is IMO unlikely to be particularly fast in terms of CPU time used per message because the per-message processing is rather complex.

With a userspace mechanism built on top of a serious IPC primitive, the extra context switch goes away because the central daemon can easily introduce parties for direct communication. Linux has no such mechanism (other than SCM_RIGHTS). seL4 does, and I suspect (although I don't know for sure) that the other L4 systems do as well. Binder also looks reasonable for such uses, even though it's rather crufty in other respects.

For dbus in particular (userspace or kernel), I think that good performance under load will be tough, because dbus has a reliable in-order broadcast model. If everyone can broadcast to everyone in order, then the overall system needs to buffer each message until every receiver has read it. Since the senders and receivers are all asynchronous, that can be a lot of buffering. For kdbus in particular, the fancy "pool" model means (AFAICT) that all of the broadcast messages need to be buffered *separately* for each receiver. IMO this will work considerably worse than just doing it with a lightweight userspace daemon. Realistically, though, the fully-ordered broadcast model seems unlikely to hold up under load with *any* implementation whatsoever.

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How Debian managed the systemd transition

Posted Sep 23, 2015 9:33 UTC (Wed) by paulj (subscriber, #341) [Link] (10 responses)

The problem is some people already went and implemented a kernel DBUS, presumably without having thought too deeply about things and not having questioned the notion that the performance problems with dbus-daemon were to do with kernel-userspace transitions. So given it exists and does improve performance over the inefficient user-space implementation, and given those people (like any others) aren't keen to have their work wasted, there will now be pressure to integrate it.

That pressure will be hard to deflect by pointing out the correct solution to an inefficient user-space implementation is not a very $FAVOURED_IPC_OF_THIS_DECADE-specific kernel implementation, but instead to implement an efficient user-space implementation + whatever generalised kernel services are needed for IPC problems in the abstract. To deflect that pressure for good requires coming up with that efficient user-space implementation really.

How Debian managed the systemd transition

Posted Sep 23, 2015 9:47 UTC (Wed) by lgeorget (guest, #99972) [Link] (2 responses)

> The problem is some people already went and implemented a kernel DBUS, presumably without having thought too deeply about things and not having questioned the notion that the performance problems with dbus-daemon were to do with kernel-userspace transitions.

Actually, if I recall correctly the discussions on that matter, the main advantage of the in-kernel implementation of dbus was not that it reduces the number of context switches but that it reduces the number of memory copies because for the kernel, unlike a user-space daemon, copying memory can be as simple as mapping the same pages in two processes.

> those people (like any others) aren't keen to have their work wasted, there will now be pressure to integrate it.

As far as I can tell from reading the mails on the Linux mailing list, Greg Kroah-Hartmann has shown to be very professional. He would surely be pleased to see his work in the mainline kernel, but not to the point to "pressure" anyone.

How Debian managed the systemd transition

Posted Sep 23, 2015 15:06 UTC (Wed) by luto (subscriber, #39314) [Link]

Indeed, kdbus saves a memory copy in the common case if the receiver is able to consume data straight from the "pool" without copying the data itself.

For small messages, this barely matters, and for large messages, both kdbus and AF_UNIX users can use memfds, which does even less copying.

Actually, for small messages, I'll only believe that the kdbus approach is faster if someone benchmarks it cleanly. The saved copy is only possible because the kernel writes to the receiver's pool when the message is sent, and that means that the kernel has to map the receiver's pool, and that's not free. (In fact it can be very slow -- modern CPUs are very good at mapping things, but at least x86 makes *unmapping* extremely expensive.)

How Debian managed the systemd transition

Posted Sep 23, 2015 15:51 UTC (Wed) by dlang (guest, #313) [Link]

Linus has pointed out that the performance wins of kdbus have far more to do with horribly inefficient userspace dbus code than any advantage of being in the kernel (context switches or memory copies)

So the 'official' justification for kdbus is no longer performance, but rather security and/or reliability

How Debian managed the systemd transition

Posted Sep 23, 2015 16:29 UTC (Wed) by raven667 (subscriber, #5198) [Link] (6 responses)

> presumably without having thought too deeply about things

I've been on the sidelines, following development on LWN, but that doesn't seem representative of the people involved or the effort which has gone into this, so I wouldn't presume that at all.

> not having questioned the notion that the performance problems with dbus-daemon were to do with kernel-userspace transitions

I believe there was awareness that the existing dbus-daemon implementation was not performant but also awareness that even a perfectly implemented userspace daemon has an upper limit on what it can do because of serializing, memory copying and context switches. Experience with the X Window protocol is instructive here as it sits in a very similar place in the software stack and there was a desire for dbus to be able to scale to the point of handling graphics data, which has already been demonstrated with X that a userspace daemon cannot do this without kernel support. Less copying and less context switches are also a boon for power usage which is becoming more important every year, both for battery powered and datacenter devices.

> efficient user-space implementation + whatever generalised kernel services are needed for IPC problems in the abstract.

This was the original goal and implementation many years ago but was flatly rejected by the kernel developers who would have needed to approve it which is why we have the kdbus implementation we have today as opposed to some other design. The original thought would be for a multicast AF_UNIX type socket that a userspace daemon could control which would be capable of zero-copy message delivery but the network subsystem maintainers refused to entertain the changes required to make something like that work and be supportable, so a different design which is much more self-contained is being proposed instead.

How Debian managed the systemd transition

Posted Oct 9, 2015 23:29 UTC (Fri) by nix (subscriber, #2304) [Link] (5 responses)

Experience with the X Window protocol is instructive here as it sits in a very similar place in the software stack and there was a desire for dbus to be able to scale to the point of handling graphics data, which has already been demonstrated with X that a userspace daemon cannot do this without kernel support.

X was doing just that without kernel support for nearly two decades. The MIT-SHM extension is worth noting.

You don't need to be the kernel to share memory... and with memfds, you don't even need to be the kernel to share memory with untrusted partners.

How Debian managed the systemd transition

Posted Oct 10, 2015 1:24 UTC (Sat) by raven667 (subscriber, #5198) [Link] (4 responses)

Shared memory is a kernel feature that gets you some of the way there but doesn't have the access control interface that these applications require and the DRI/DRM interfaces in the kernel were created for graphics applications like X, much like memfd which was created for kdbus, so I don't think its fair to say that X runs undegraded without special kernel support.

How Debian managed the systemd transition

Posted Oct 13, 2015 13:50 UTC (Tue) by nix (subscriber, #2304) [Link] (3 responses)

What? X ran undegraded without kernel support for literally a decade plus, until hardware 3D stuff started turning up. MIT-SHM provided everything needed.

How Debian managed the systemd transition

Posted Oct 13, 2015 14:45 UTC (Tue) by nybble41 (subscriber, #55106) [Link] (2 responses)

> X ran undegraded without kernel support for literally a decade plus

I think one could argue that being given direct access to the graphics hardware, and thus effectively unlimited access to the entire system, should count as "kernel support". Sure, the driver code was inside the X server rather than compiled into the kernel or a loadable module, but it still required special interfaces used primarily by X, and it wasn't possible to run the X server as an ordinary, non-root user process.

How Debian managed the systemd transition

Posted Oct 13, 2015 15:09 UTC (Tue) by raven667 (subscriber, #5198) [Link] (1 responses)

That's a good point, but even if you don't consider allowing the userspace app to just bang away at /dev/mem "kernel support" because that really isn't a defined API, certainly we say that limiting to the performance and capabilities of the 1990's X stack would be considered "degraded" by modern standards and applications. Making this behave safely without degraded performance required the addition of dedicated APIs, to talk to the graphics co-processor, to share memory buffers, beyond the 1980's UNIX standard ones.

We've already gone down the route of adding dedicated IPC APIs for SysV, for Netlink, for X/Wayland and now for DBUS, which I see as following the evolution of OS design and the needs of the applications of the era when these interfaces were designed.

How Debian managed the systemd transition

Posted Oct 13, 2015 22:49 UTC (Tue) by nix (subscriber, #2304) [Link]

Oh, I agree it would be bad by modern standards -- however, it was quite clearly capable of scaling to the point of handling graphics data with no more kernel support than that. To get back to the original point: unless you think D-Bus is not just going to be asked to handle graphics data but the full graphics flow of a 3D game I think the volume of data involved in graphics should not serve as an argument for needing kernel support just to handle that.

How Debian managed the systemd transition

Posted Sep 25, 2015 22:24 UTC (Fri) by oak (guest, #2786) [Link]

Yea, the buffering is much larger performance issue than context switching. All it takes is some message that is generated very frequently, and a client that has subscribed to the message, but isn't reading its messages (e.g. because it's suspended for few days while on background).

Result is that daemon message buffers grow until they take all your memory, your system message transport goes to swap (with everything else) and things become *really* slow until the problematic client is killed. If the client is woken up, daemon and client can spend many minutes (or hours depending on how much swap & buffering you have) during which bus isn't very responsive. If allocations were mixed well enough, emptying the message buffer on daemon doesn't actually free its dirtied memory because it's gotten fragmented.

This is D-BUS experience from 5-10 years ago on semi-embedded device. Even worse, the user-space daemon gets it's memory fragmented very easily and doesn't return to system memory it's once allocated. So, local DOS is trivial to do with any client that can connect to bus.

Some of the things where kernel *might* be able to improve on this are:
* Assigning message buffers memory cost to corresponding client, so that admin can identify who's the culprit
* Better allocator that guarantees that after processing the messages, the emptied buffer can actually be freed for other purposes (i.e. allocation blocks don't mix data with unrelated life-times, e.g. send and receive messages or messages from/to different clients)
* If message is status broadcast, maybe having some mechanism where only last status update is buffered
* Suspending message sending if receiver isn't processing the messages