Re: [PATCH v3 4/4] mm: Adaptive hash table scaling

[Andrew Morton <akpm@linux-foundation.org>]

On Thu,  2 Mar 2017 00:33:45 -0500 Pavel Tatashin <pasha.tatashin@oracle.com> wrote:

> Allow hash tables to scale with memory but at slower pace, when HASH_ADAPT
> is provided every time memory quadruples the sizes of hash tables will only
> double instead of quadrupling as well. This algorithm starts working only
> when memory size reaches a certain point, currently set to 64G.
> 
> This is example of dentry hash table size, before and after four various
> memory configurations:
> 
> MEMORY	   SCALE	 HASH_SIZE
> 	old	new	old	new
>     8G	 13	 13      8M      8M
>    16G	 13	 13     16M     16M
>    32G	 13	 13     32M     32M
>    64G	 13	 13     64M     64M
>   128G	 13	 14    128M     64M
>   256G	 13	 14    256M    128M
>   512G	 13	 15    512M    128M
>  1024G	 13	 15   1024M    256M
>  2048G	 13	 16   2048M    256M
>  4096G	 13	 16   4096M    512M
>  8192G	 13	 17   8192M    512M
> 16384G	 13	 17  16384M   1024M
> 32768G	 13	 18  32768M   1024M
> 65536G	 13	 18  65536M   2048M

OK, but what are the runtime effects?  Presumably some workloads will
slow down a bit.  How much? How do we know that this is a worthwhile
tradeoff?

If the effect of this change is "undetectable" then those hash tables
are simply too large, and additional tuning is needed, yes?

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