A timerless CLOCK algorithm

I am writing this down partly as a way of seeing if it makes sense…

Minor faults we can live with: if we have to reaffix a mapping back into the TLB it takes a few cycles but that is not a heavy burden. Major faults, however, are a different matter: they take many thousands of processor cycles to fix and have to be avoided at all costs.

Yet we have a basic problem in that we cannot mark each and every page access with a timestamp, so we have to guess at which page was least recently used – and so typically we use the sweeping hand of a CLOCK algorithm to do this.

But I have an even bigger problem – I have no time source, and hence no timer interrupts, to use.

So instead I want to try this:

  • All entries in the page table (PT) will have two validity bits, one we call VALID (V) and another AVAILABLE (A);
  • When a page is first mapped in the PT, both V and A will be marked as on.
  • When we first run out of space in the PT we will start to advance through the PT on each access turning off a V bit. So, if we pin the first three entries in the PT (eg for ‘kernel’ code, the PT itself and a stack), the first time we go round we might mark V off for the 16 entries in the PT from 2 – 18
  • Then, when we are mapping a page for which no mapping exists, we take the first entry where V is off
  • But before that we check whether we have any entries where A is on that map our page, in which case we switch V on for that mapping and we are off

 

Having written that out I can see we only need the A bit if we think that a 0x00 <–> 0x00 mapping might be a valid one, but we don’t have to make that assumption and so we can restrict this to just the V bit.