Re: [Qemu-devel] [PATCH 13/16] translate-all: protect TB jumps with a per-destination-TB lock

[Paolo Bonzini]

On 27/02/2018 06:39, Emilio G. Cota wrote:
> Using a hash table or a binary tree to keep track of the jumps
> doesn't really pay off, not only due to the increased memory usage,
> but also because most TBs have only 0 or 1 jumps to them. The maximum
> number of jumps when booting debian-arm that I measured is 35, but
> as we can see in the histogram below a TB with that many incoming jumps
> is extremely rare; the average TB has 0.80 incoming jumps.
> 
> n_jumps: 379208; avg jumps/tb: 0.801099
> dist: [0.0,1.0)|▄█▁▁▁▁▁▁▁▁▁▁▁ ▁▁▁▁▁▁ ▁▁▁  ▁▁▁     ▁|[34.0,35.0]

This makes sense, for example:

   while(...) {
   }

2 basic blocks, 0 and 1 incoming jumps (avg 0.5)

   if(...) {
   }

2 basic blocks, 0 and 1 incoming jumps (avg 0.5)

   if(...) {
   } else {
   }

3 basic blocks, 0, 1 and 1 incoming jumps (avg 0.66)

So 0.8 is actually a lot. :)  The long tail is probably for switch
statements.

Paolo