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From: Laurent Desnogues <laurent.desnogues@gmail.com>
Date: Tue, 27 Feb 2018 12:43:59 +0100
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Subject: Re: [Qemu-devel] [PATCH 13/16] translate-all: protect TB jumps with
 a per-destination-TB lock
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Emilio G. Cota" <cota@braap.org>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, Richard Henderson <richard.henderson@linaro.org>

On Tue, Feb 27, 2018 at 12:33 PM, Paolo Bonzini <pbonzini@redhat.com> wrote=
:
> 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)|=E2=96=84=E2=96=88=E2=96=81=E2=96=81=E2=96=81=E2=96=81=
=E2=96=81=E2=96=81=E2=96=81=E2=96=81=E2=96=81=E2=96=81=E2=96=81 =E2=96=81=
=E2=96=81=E2=96=81=E2=96=81=E2=96=81=E2=96=81 =E2=96=81=E2=96=81=E2=96=81  =
=E2=96=81=E2=96=81=E2=96=81     =E2=96=81|[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.

And calls too :-)


Laurent

> Paolo
>