GoKawiilRCU has a way of popping up unexpectedly. In the words of Fedor Pikus, “In fact, you may already be using the RCU approach in your program without realizing it!” This post will describe uses of a couple of highly unorthodox (some might say “completely irresponsible”) types of RCU implementations, both accidental and otherwise.
Timed-Wait RCU
One very simple class of RCU implementations uses a fixed period of time for the grace period. This can clearly work well in hard real-time kernels and applications, though it has also been used in a prototype non-real-time kernels. In an early 1990s verbal discussion, none other than Van Jacobson pointed out that a 15-second delay would suffice in the research version of a proprietary-UNIX OS that he was working with. I responded (also verbally) that in DYNIX/ptx interrupt handlers sometimes executed for more than a minute (as in more than 15 seconds), but that Jack Slingwine and I had a way to get the same low-overhead/high-scalability effect without the need for hard real-time constraints on readers. Van expressed interest, so I sent him an early draft of the first RCU conference paper. Some years later, I had the privilege of hearing Van say nice things about Linux-kernel RCU.
For the benefit of any long-time RCU users who (like me) have a “make it make sense” filter in their heads, here is a Linux-kernel implementation of a synchronize_rcu() for Van's RCU:
void synchronize_rcu(void) { schedule_timeout_uninterruptible(15 * HZ); }
In other words, synchronize_rcu() does a fixed wait of 15 seconds, after which it is assumed, without proof, that all pre-existing readers have completed.
In the mid-1990s, Aju John wrote the USENIX paper Dynamic vnodes — Design and Implementation, which proposed a fixed 10-minute wait time for reclaiming vnodes in a proprietary UNIX system, DEC OSF/1 Version 3.0.
This approach might actually make sense in a hard-real-time environment, but would of course be extremely dangerous even there. On the other hand, you have to admit that Van and Aju were taking a no-holds-bared approach to performance and scalability, and doing so very early in the game! And Paul Khuong reports that this technique was recently used in production for an extended period, until a more principled and sufficiently performant technique was put in its place.
As far as I know, none of these timed-wait RCU use cases remains in production use, but the fact remains that timed-wait RCU really has been used in production. And maybe it is still being used, dangerous though use of timed-wait RCU might be outside of hard-real-time environments! On the other hand, this is a rare instance of hard real-time making something way simpler, at least as long as non-real-time threads are prohibited from using RCU read-side critical sections.
Fixed-Buffer RCU
... continue reading