operating-system,virtual-memory,context-switch,tlb,page-tables , Flush TLB on a context swtich


Flush TLB on a context swtich

Question:

Tag: operating-system,virtual-memory,context-switch,tlb,page-tables

This may depends on the OS, but in general as I understand that when there a page fault (the desired page is not in main memory) occurs OS will instruct CPU to read the page from disk, and I am wondering does OS dispatch to another process while the disk I/O ? if it does then there will be a complete flushing of the TLB on a context switch, correct ?


Answer:

More or less, but a page fault doesn't always mean the page is on the disk (it could also not exist at all, be a lazy-allocation page, be a copy-on-write page that was written to, exist but be marked unreadable/unwritable, etc). But if that's how it is, it's probably going to schedule an other thread at least because disk IO takes approximately forever.

The amount of switching necessary depends on what it switches to, switching between threads from the same context doesn't imply a TLB flush. If a TLB flush is necessary, it's probably not a complete flush, because of global pages (so typically, you're not flushing out TLB entries for kernel pages). There is also PCID to avoid complete flushes (flushing can be limited to specified process context IDs), but that's quite recent, and tricky to use since there are only 4096 different IDs.


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