6 ms·

On linux, system calls don't result in a TLB flush - kernel data structures and code are in a different portion of the virtual address space (starting from the

• gary_bernhardt 12y ago
  I didn't know that! It certainly makes sense. Context switches still thrash the TLB, though. The performance cost of that has gotten better as time has gone on, but I wonder how many transistors (and how much power) CPUs are burning for that mitigation. The "how computers actually work" digression originally had a section on context switches, but I removed it early on because I felt like that section was dragging. To try to paint a very rough picture of the larger thoughts from which this talk was taken: I think that microkernels and the actor model are both the right thing (most of the time). When implemented naively, they both happen to take a big penalty from context switch cost. But Erlang can host a million processes in its VM, and we're using VMs for almost everything now anyway. The obvious (to me) solution is to move both the VM and an Erlang-style, single-address-space scheduler into the kernel. Then you can have a microkernel and a million native processes without the huge overhead of naive implementations. There are surely many huge practical hurdles to overcome with that, and maybe some that can't be overcome at all, but it sure sounds right when written in two paragraphs. ;)
  • deleted 12y ago
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  • nly 12y ago
    What you seem to be missing with re: asm.js is that, while the JIT to native code gets you your super-fast integer operations, it's still critically incomplete with regard to memory access. Every single individual memory access has to be bounds checked or pushed through some other indirection inside the runtime. Google demonstrated similar ideas with NaCl, which achieved safety with a similarly restricted native code and a just-in-time verification step. Even if these memory accesses could be made as efficient as those performed by the CPUs access protection, you're still not gaining anything you don't already have. Regarding context switches: A full CPU context switch on x86 (not to ring1 but between two arbitrary points within a single userland address space) takes a few dozen instructions and about 40-80 cycles. A single cache-line miss resulting in a load from main memory on the other hand takes at least twice that (~200 cycles). Again, hits from jumping around in memory will dominate. How significant is a 20% overhead from virtual memory? Probably about the same as getting 1% more of your memory accesses back in to high level caches.
    • andhow 12y ago
      On x64 in Firefox, at least, there are no bounds checks; the index is a uint32; the entire accessible 4GB range is mapped PROT_NONE with only the accessible region mapped PROT_READ|PROT_WRITE; out-of-bounds accesses thus reliably turn into SIGSEGVs which are handled safely after which execution resumes. Thus, bounds checking is effectively performed by the MMU.
      • nly 12y ago
        Interesting approach, might have to look at the code. Nonetheless it highlights how useful the MMU is and how none of this is free.
    • cornholio 12y ago
      I agree, and I think the whole premise of the performance gain is based on a "have the cake and eat it too" fallacy. Sure, the virtualized syscall to the virtualized OS will be free, but the painting of the font on the screen or the reading of the socket data will be done by the actual bare metal OS which the VM will invoke to get the actual job done. So as long as we are talking about interprocess communication there will be a gain, but not for the actual hardware facing operation. Then again, you are trading a hardware enforced isolation which is simple and proven for a isolation enforced by a complex and fragile VM.
  • jerf 12y ago
    You know about http://erlangonxen.org/ http://erlangonxen.org/ ? Also something like http://corp.galois.com/halvm http://corp.galois.com/halvm . That's probably still not quite low enough level to turn off the MMU, but it's getting there.