4 ms·

> There is a good reason to page- or superpage-align code generally; it burns some virtual memory but reduces TLB overhead and therefore misses / invalidations,

> There is a good reason to page- or superpage-align code generally; it burns some virtual memory but reduces TLB overhead and therefore misses / invalidations, which are very costly. You would want to do the same with executable code in a static-linked binary. But most code isn't performance critical. Thus trying to align functions to page boundaries is just wasting memory. Even in performance critical code, aligning to cache line sizes is enough and aligning to page boundaries doesn't provide any advantage.

MaulingMonkey 6y ago
Not only is it wasting memory, but I wouldn't be convinced it'd not be outright hurting performance by increasing cache line aliasing, increasing TLB overhead and misses/invalidations! Avoiding page alignment can be a performance gain! https://pvk.ca/Blog/2012/07/30/binary-search-is-a-pathological-case-for-caches/ https://pvk.ca/Blog/2012/07/30/binary-search-is-a-pathologic... Like, sure, mapping your executable's code section in at a page boundary is probably fine, but I think trying to align individual functions to page boundaries would be a counterproductive mistake as a general strategy.
- loeg 6y ago
  > I think trying to align individual functions to page boundaries would be a counterproductive mistake as a general strategy. Yes — which is why no one does that. I don't know where bjourne came up with the idea.
loeg 6y ago
> But most code isn't performance critical. Wasting TLB slots on your unimportant code still pessimizes your hot code. > Thus trying to align functions to page boundaries is just wasting memory. No one aligns individual functions to page boundaries; you align the entire loadable code segment to a page (or preferably, superpage) boundary. > Even in performance critical code, aligning to cache line sizes is enough and aligning to page boundaries doesn't provide any advantage. This is a different kind of optimization (avoiding cache line contention) than I was talking about (optimizing TLB slot use). Yes, forced page-alignment doesn't help cache line contention anymore than forced cacheline-alignment. But that's irrelevant for code, generally: (outside of self-modifying code, which is extremely uncommon) code doesn't share a cacheline with memory that will be mutated and thus doesn't contend in that way.
- bjourne 6y ago
  Then I don't know what you're getting at. Clearly the most efficient approach would be fitting your performance critical loop into one or at most two consecutive pages making the TLB a moot point.

MaulingMonkey 6y ago

Not only is it wasting memory, but I wouldn't be convinced it'd not be outright hurting performance by increasing cache line aliasing, increasing TLB overhead and misses/invalidations! Avoiding page alignment can be a performance gain! https://pvk.ca/Blog/2012/07/30/binary-search-is-a-pathological-case-for-caches/ https://pvk.ca/Blog/2012/07/30/binary-search-is-a-pathologic... Like, sure, mapping your executable's code section in at a page boundary is probably fine, but I think trying to align individual functions to page boundaries would be a counterproductive mistake as a general strategy.

loeg 6y ago
> I think trying to align individual functions to page boundaries would be a counterproductive mistake as a general strategy. Yes — which is why no one does that. I don't know where bjourne came up with the idea.

loeg 6y ago

> But most code isn't performance critical. Wasting TLB slots on your unimportant code still pessimizes your hot code. > Thus trying to align functions to page boundaries is just wasting memory. No one aligns individual functions to page boundaries; you align the entire loadable code segment to a page (or preferably, superpage) boundary. > Even in performance critical code, aligning to cache line sizes is enough and aligning to page boundaries doesn't provide any advantage. This is a different kind of optimization (avoiding cache line contention) than I was talking about (optimizing TLB slot use). Yes, forced page-alignment doesn't help cache line contention anymore than forced cacheline-alignment. But that's irrelevant for code, generally: (outside of self-modifying code, which is extremely uncommon) code doesn't share a cacheline with memory that will be mutated and thus doesn't contend in that way.

bjourne 6y ago
Then I don't know what you're getting at. Clearly the most efficient approach would be fitting your performance critical loop into one or at most two consecutive pages making the TLB a moot point.