Re: [RFC PATCH V4] mm readahead: Fix readahead fail for no localmemory and limit readahead pages

From: Raghavendra K T
Date: Thu Jan 16 2014 - 06:16:19 EST

On 01/10/2014 03:22 PM, Jan Kara wrote:
On Fri 10-01-14 09:36:56, Jan Kara wrote:
On Fri 10-01-14 00:54:50, Raghavendra K T wrote:
We limit the number of readahead pages to 4k.

max_sane_readahead returns zero on the cpu having no local memory
node. Fix that by returning a sanitized number of pages viz.,
minimum of (requested pages, 4k, number of local free pages)

fadvise experiment with FADV_WILLNEED on a x240 machine with 1GB testfile
32GB* 4G RAM numa machine ( 12 iterations) yielded

kernel Avg Stddev
base 7.264 0.56%
patched 7.285 1.14%
OK, looks good to me. You can add:
Reviewed-by: Jan Kara <jack@xxxxxxx>
Hum, while doing some other work I've realized there may be still a
problem hiding with the 16 MB limitation. E.g. the dynamic linker is
doing MADV_WILLNEED on the shared libraries. If the library (or executable)
is larger than 16 MB, then it may cause performance problems since access
is random in nature and we don't really know which part of the file do we
need first.

I'm not sure what others think about this but I'm now more inclined to a
bit more careful and introduce the 16 MB limit only for the NUMA case. I.e.
something like:

Hi Linus, Andrew,

Could you please let us know your suggestion or comment?

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