LRU queue strangeness

Prinzipdarstellung der Arbeitsweise einer MMU

(Photo credit: Wikipedia)

For the last week or so I have been writing and then debugging (and so mainly debugging) a least-recently-used (LRU) page replacement system on my Microblaze simulation.

Perhaps I shouldn’t have bothered – I had a working first-in-first-out (FIFO) system after all. But no one seriously uses FIFO, so I had to write some LRU code.

I thought I had it working tonight – it ran through the target exercise in about 6 million instructions (as the MMU in a Microblaze is crude, memory gets loaded in and out ‘by hand’ and so the instruction count measures time/efficiency) when the system had 32 4k local pages and in about 10.5 million instructions when it had 24 4k pages available locally – all seems fine: less pages means more time is required.

But then things started to get weird – testing the system with 28 pages took about 9 million instructions, but when I tried to use 26 pages I had to break the code after it had executed 14 trillion instructions.

Indeed it seems to only work for a very limited number of page counts. How odd – though a typically debuggers story. A week in, finally thinking I’d cracked it when some really strange behaviour manifests itself.

Update: It appears to be an unaligned data exeception issue. Somewhere along the line a piece of code relies on the LRU queue to be a multiple of 4 in length would be my guess…