Re: [Qemu-devel] [PATCH v3 11/11] translate-all: add tb hash bucket info to 'info jit' dump

[Emilio G. Cota]

On Sun, Apr 24, 2016 at 18:06:51 -0400, Emilio G. Cota wrote:
> On Sun, Apr 24, 2016 at 12:46:08 -0700, Richard Henderson wrote:
> > On 04/22/2016 04:57 PM, Emilio G. Cota wrote:
> > >On Fri, Apr 22, 2016 at 12:59:52 -0700, Richard Henderson wrote:
> > >>FWIW, so that I could get an idea of how the stats change as we improve 
> > >>the
> > >>hashing, I inserted the attachment 1 patch between patches 5 and 6, and 
> > >>with
> > >>attachment 2 attempting to fix the accounting for patches 9 and 10.
> > >
> > >For qht, I dislike the approach of reporting "avg chain" per-element,
> > >instead of per-bucket. Performance for a bucket whose entries are
> > >all valid is virtually the same as that of a bucket that only
> > >has one valid element; thus, with per-bucket reporting, we'd say that
> > >the chain lenght is 1 in both cases, i.e. "perfect". With per-element
> > >reporting, we'd report 4 (on a 64-bit host, since that's the value of
> > >QHT_BUCKET_ENTRIES) when the bucket is full, which IMO gives the
> > >wrong idea (users would think they're in trouble, when they're not).
> > 
> > But otherwise you have no way of knowing how full the buckets are.  The
> > bucket size is just something that you have to keep in mind.
> 
> I'll make some changes in v4 that I think will address both your and
> my concerns:
> - Report the number of empty buckets
> - Do not count empty buckets when reporting avg bucket chain length
> - Report average bucket occupancy (in %, so that QHT_BUCKET_ENTRIES
>   does not have to be reported.)

How does the following look?

Example with good hashing, i.e. func5(phys_pc, pc, flags):
TB count            704242/1342156
[...]
TB hash buckets     386484/524288 (73.72% used)
TB hash occupancy   32.57% avg chain occupancy. Histogram: 
0-10%▆▁█▁▁▅▁▃▁▁90-100%
TB hash avg chain   1.02 buckets. Histogram: 1█▁▁3

Example with bad hashing, i.e. func5(phys_pc, 0, 0):
TB count            710748/1342156
[...]
TB hash buckets     113569/524288 (21.66% used)
TB hash occupancy   10.24% avg chain occupancy. Histogram: 
0-10%█▁▁▁▁▁▁▁▁▁90-100%
TB hash avg chain   2.11 buckets. Histogram: 1▇▁▁▁▁▁▁▁▁▁93

Note that:

- "TB hash avg chain" does _not_ count empty buckets. This gives
  an idea of how many buckets a typical hit goes through.

- "TB hash occupancy" _does_ count empty buckets. It is called
  "avg chain occupancy" and not "avg occupancy" because the
  counts are only valid per-chain due to the seqlock protecting
  each chain.

Thanks,

                Emilio