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List:       openbsd-tech
Subject:    Re: shrinking and growing reallocs: a theoretical? bad case for performance
From:       Otto Moerbeek <otto () drijf ! net>
Date:       2020-08-31 18:15:46
Message-ID: 20200831181546.GD95336 () clue ! drijf ! net
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On Mon, Aug 31, 2020 at 11:25:51AM -0600, Theo de Raadt wrote:

> > Taking advantage of the sparse address space is smart and as 64-bit
> > is now the norm, that space is even sparser.
> 
> Fundamentally this is moving various forms of pressure to the kernel,
> which does not do the best job yet.

This effect is reduced by making small shrinks a no-op.

> 
> The pivot code in mmap for new mappings isn't entirely bug-free so we've
> avoided it turning it on.  The idea of that code is be random as
> neccessary -- creating "unknowable addresses", but in doing so avoid
> fragmenting the address space excessively.  Excessive fragmentation in turn
> fragmentations allocation in multi-level page-tables, and that in turn
> results in excessive TLB pressure.  Which is difficult to gauge since things
> keep working, but brings in a big performance cost.
> 
> Basically we were brave to do very high amounts of randomization early on.
> At a cost.  But our work to improve the cost isn't finished.

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