Wu Fengguang wrote:
> When lifting the default readahead size from 128KB to 512KB,
> make sure it won't add memory pressure to small memory systems.
>
> For read-ahead, the memory pressure is mainly readahead buffers consumed
> by too many concurrent streams. The context readahead can adapt
> readahead size to thrashing threshold well. So in principle we don't
> need to adapt the default _max_ read-ahead size to memory pressure.
>
> For read-around, the memory pressure is mainly read-around misses on
> executables/libraries. Which could be reduced by scaling down
> read-around size on fast "reclaim passes".
>
> This patch presents a straightforward solution: to limit default
> readahead size proportional to available system memory, ie.
> 512MB mem => 512KB readahead size
> 128MB mem => 128KB readahead size
> 32MB mem => 32KB readahead size (minimal)
>
> Strictly speaking, only read-around size has to be limited. However we
> don't bother to seperate read-around size from read-ahead size for now.
>
> CC: Matt Mackall <mpm@xxxxxxxxxxx>
> Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
What I state here is for read ahead in a "multi iozone sequential"
setup, I can't speak for real "read around" workloads.
So probably your table is fine to cover read-around+read-ahead in one
number.
I have tested 256MB mem systems with 512kb readahead quite a lot.
On those 512kb is still by far superior to smaller readaheads and I
didn't see major trashing or memory pressure impact.
Therefore I would recommend a table like:
>=256MB mem => 512KB readahead size
128MB mem => 128KB readahead size
32MB mem => 32KB readahead size (minimal)
--
Grüsse / regards, Christian Ehrhardt
IBM Linux Technology Center, System z Linux Performance
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