[PATCH] Configure: Fix cacheline size for aarch64 platforms

[Maxim Dounin]

Hello!

On Fri, Dec 08, 2017 at 06:43:15PM +0000, debayang.qdt wrote:

> Hello,
> 
> 64 bytes should be the minimum size.

Any links to support this claim?

> However, A53 is a small segment of arm64 based servers . 
> Multiple aarch64 based servers uses 128 byte cache lines.
> For e.g  Centriq 2400 aarch64 server from Qualcomm Datacenter Technologies uses 128 bytes cache line.

AFAIK, using a smaller cache line size is mostly fine and imply 
almost no performance difference in nginx.  On ther other hand, 
using larger cache line size can easily result in multiple memory 
access where just one should be enough, resulting in suboptimal 
performance.  That's why we generally assume 32 unless we know 
better.

> To be very accurate , we may read the dcache line size from CTR_EL0 register.
> Let me know what you think.

Given the level of popularity of aarch64 servers, I would rather 
not.  I would prefer to rely on something architecture-independent 
like sysconf(_SC_LEVEL1_DCACHE_LINESIZE), though it looks like it 
doesn't work on aarch64 yet, see 
https://bugzilla.redhat.com/show_bug.cgi?id=1190638.

-- 
Maxim Dounin
http://mdounin.ru/