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0 pointssmegel9y ago0 comments

You say "hey process, this data is not available yet, but if you listen on this event port I will sent you a notification when it is ready, then you can do whatever you want with it. In the meantime, continue to serve up data that is in memory".

Pretty simple stuff really.

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6 comments · 1 top-level

smarnach9y ago· 5 in thread

You might be able to do that in response to a system call asking whether a memory page is available, but not in response to a page fault. A page fault occurs if the process tries to access a virtual memory page that's not currently mapped to physical memory. Unless I'm missing something, the only reasonable way of resuming the process at the machine code instruction that caused the page fault is to first make sure the memory is available. Otherwise you will simply get another page fault.

dllthomas9y ago

You don't necessarily need to resume the process at the machine code instruction that caused the page fault.

smarnach9y ago

Right, you also have the option of not resuming it at all.

1 more reply

smegelOP9y 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?

smarnach9y 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.

1 more reply

twic9y 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.

j / k navigate · click thread line to collapse

0 comments

6 comments · 1 top-level

smarnach9y ago· 5 in thread

dllthomas9y ago

You don't necessarily need to resume the process at the machine code instruction that caused the page fault.

smarnach9y ago

Right, you also have the option of not resuming it at all.

1 more reply

smegelOP9y ago

1. thread: hey kernel, I would like to read this page of memory.

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.

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?

smarnach9y ago

1 more reply

twic9y 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.

j / k navigate · click thread line to collapse