How Debian managed the systemd transition [LWN.net]

How Debian managed the systemd transition

Posted Sep 23, 2015 9:47 UTC (Wed) by lgeorget (guest, #99972)
In reply to: How Debian managed the systemd transition by paulj
Parent article: How Debian managed the systemd transition

> 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.

to post comments

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