4 ms·

1. thread: hey kernel, I would like to read this page of memory. 2. kernel: hey thread, that page is still on SLOOOOW spinning disk, why don't you go off and d

by smegel 10y ago

1. thread: hey kernel, I would like to read this page of memory. 2. kernel: hey thread, that page is still on SLOOOOW spinning disk, why don't you go off and do something else while I get it for you. I'll let you know with an event notification, so be sure to check in with kqueue regularly. 3. thread: OK then, i'll go off and serve these other people while you do that for me. kthanx. 4. kernel: hey DMA controller, I'd like you to get sectors 4, 5 and 6 from platter 3 on HDD 2 and load them into memory address 0x4fe6bb. Please send me an interrupt when done. 5. DMA controller: OK servo, please adjust read head to this offset. Read head, read me those bytes. Memory controller, please store bytes at address 0x4fe6bb. Hey CPU, here's an interrupt to store in your interrupt table, please wake up the kernel guy and let him know. 6. kernel: wow I just got interrupted. The interrupt seems to map to a request for data from this particular thread. Better let him know. (sends event up thread's kqueue). 7. thread: hey, I just got a kqueue notification that the file is now ready to be read. That means it must be in memory...cool! Make sense now?

smarnach 10y ago
I get that you can do this in response to a system call (the one issued in step 1). You could avoid blocking in Linux as well using madvice() and mincore(), as pointed out in the comments to the article. However, once you get an actual page fault, the only option is to block the process.
- smegel 10y ago
  > I get that you can do this in response to a system call (the one issued in step 1). You could avoid blocking in Linux as well using madvice() and mincore() Yes, but the one thing they don't give you is notification the data is in memory. Do you really want to spin a hot loop calling mincore? > However, once you get an actual page fault, the only option is to block the process. See steps 1-3 again.
  - lgunsch 10y ago
    The only way to receive an event would be from a signal, like SIGSEGV. There is no way to hook a jump instruction up to kqueue. In any case, this would mean writing specific code to handle the case, implying it is not simply a difference of the FreeBSD kernel vs the Linux kernel.
twic 10y ago
This is roughly what 'scheduler activations' and 'kernel scheduled entities' did. Except rather than just being in response to page faults, it was any interaction with the kernel that would block.

by smegel 10y ago

smarnach 10y ago

I get that you can do this in response to a system call (the one issued in step 1). You could avoid blocking in Linux as well using madvice() and mincore(), as pointed out in the comments to the article. However, once you get an actual page fault, the only option is to block the process.

smegel 10y ago
> I get that you can do this in response to a system call (the one issued in step 1). You could avoid blocking in Linux as well using madvice() and mincore() Yes, but the one thing they don't give you is notification the data is in memory. Do you really want to spin a hot loop calling mincore? > However, once you get an actual page fault, the only option is to block the process. See steps 1-3 again.
- lgunsch 10y ago
  The only way to receive an event would be from a signal, like SIGSEGV. There is no way to hook a jump instruction up to kqueue. In any case, this would mean writing specific code to handle the case, implying it is not simply a difference of the FreeBSD kernel vs the Linux kernel.

twic 10y ago

This is roughly what 'scheduler activations' and 'kernel scheduled entities' did. Except rather than just being in response to page faults, it was any interaction with the kernel that would block.