Comment by temac
7 years ago
About shared libraries, I know that there is this line of thought considering them "evil" (well at least sufficiently to want to get rid of them); but I'm quite unsure about what a modern system would look like without them (although this is less a problem at the application level on e.g. Android, the system level is still extremely important)
With Spectre, proper process bounds (well, address spaces) are more important than ever -- and oh well even without that I'd still have cited them as incredibly important, in the sense that I'd rather have more than fewer. Given that, code reuse involves shared libraries, for several good reasons; the obvious one being not wasting RAM, but then there is the update problem (how to patch programs when security holes are discovered, especially if multiple parties are involved), and on top of that there is the cache pollution problem, which is related to the code duplication problem, and which is quite insidious because it is probably simultaneously hard to benchmark and very real (ambient loss of perf, just not in very hot paths, but this will still have an impact on the general perf of a system, quite like Spectre mitigations are having a big impact)
Now we could like address space boundaries so much that we would want to just use even MORE processes in place of shared libraries, but this obviously does not work for all services (and Spectre is biting us again because context switches are not cheap), plus if you take it to the extreme this makes systems extremely hard to design, and even bigger. This is part of the reasons we are using Linux instead of Hurd... (well Linux is too much in the opposite direction, but there are hopes that it will in the long term evolve toward a middle ground)
And anyway that does not fit the narrative at all of using more huge pages.
Now there are the usual radical ideas about how everything should be running on some kind of VM (sometimes even including the kernel), drastically reducing the amount of "native" code; but given the reality of our current systems that "everything" both relies on multiple VMs and I doubt it will tend to only one, nor should it (because of the monoculture this would induce). Plus the ambient perfs are still lower than native code, and TBH I don't expect that to change ever.
So, why and how would you like to get rid of shared libraries?
We are using Linux instead of Hurd due to manpower.
Most high integrity real time OSes are microkernels.
Interesting that you mention Android, one of the key points of Project Treble is using separate processes for drivers with Android IPC to talk to the kernel (including hardware buffer handles).
Well in the end we are using any X rather than Y tech because of manpower, regardless of pretty much any other characteristics.
So let's put manpower kind of aside for the bulk of the dev (where thousands of man-years are needed for any big project) and look at what could actually be achieved with the very small amount of manpower bootstrapping those projects. At this point you understand that the manpower thing is only a convenient narrative, while the reality is that even at the early time, Linux based systems worked really better than Hurd based systems.
Because general purpose micro-kernels based systems are hard, and especially those with a design as ambitious as the Hurd. (When you start to want to strongly isolate FS from VM code, it even stops being just hard and starts to be really HARD.)
And this was even worse at the time for perf reasons (but perf reasons are still applicable even today, given the impact on mobile and datacenter workloads)
However, yes, I'm in favor of more isolation today, because for a shitload ton of drivers it literally won't make any difference whether or not you take 1us vs 50us if you need to execute once every few seconds. So it is retarded to the highest level to run in kernel space if you don't actually need it. Sadly, Linux is way behind on that subject today.
That being said, and back to the original subject, a microkernel or at least a less monolithic one won't really get us in the less shared-libraries direction if it just re-implements the same perimeter of features of a monolithic ones, nor in the huge pages everywhere direction...