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0 pointsjstimpfle7y ago0 comments

> Except memory is virtual. Memory location 0x1000 might be forward, or backwards compared to 0xFFF, depending on the state of the Translation-lookaside buffer (TLB).

Does that matter though? I would assume that the "prefetcher" (or whatever it's called) can make its predictions in terms of virtual memory.

Regarding the linked list, it has been common wisdom for a long time that one should prefer sequential memory instead of linked lists. A nice benefit is that this simplifies the code as well :-). Growing and reallocating sequential buffers might not be possible in very dynamic/real-time and decentralized architectures - like a kernel - though.

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dragontamer7y ago

> Does that matter though?

Yeah. Meltdown means that the OS (as a security measure) wipes away the TLB whenever you make a system call. So all of those cached TLB entries disappear every syscall due to Kernel Page-Table Isolation.

And then the CPU core has to start from scratch, rebuilding the TLB-cache again.

So last year (when we didn't know about Meltdown), a system-call was fast and efficient. This year, on Intel and ARM systems (vulnerable to Meltdown), system-calls are now forced to wipe TLB. (But not on AMD-systems, which happened to be immune to the problem)

Both AMD and Intel implement x86 instruction set, and now the performance characteristics is different between the two boxes for something as simple as "blah = blah->next".

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The important bit is that C is still quite "high level", and indeed, even Assembly language "lies" to the programmer through virtual memory. The OS (specifically page-tables) can interject and have some magic going on even as assembly-code looks up specific addresses.

The simple pointer-dereference *blah is actually incredibly complicated. There's no real way to know its performance characteristics from a C-level alone. It depends on the machine, the OS, the configuration of the OS (ie: MMap, Swap, Huge Page support, Meltdown...) and more.

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