linux-kernel - Re: [PATCH] arm64: Increase the max granular size

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Date:	Fri, 25 Sep 2015 16:45:16 +0200
From:	Robert Richter <robert.richter@...iumnetworks.com>
To:	Will Deacon <will.deacon@....com>
CC:	Robert Richter <rric@...nel.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Tirumalesh Chalamarla <tchalamarla@...ium.com>
Subject: Re: [PATCH] arm64: Increase the max granular size

Will,

On 22.09.15 19:29:02, Will Deacon wrote:
> On Tue, Sep 22, 2015 at 06:59:48PM +0100, Robert Richter wrote:
> > From: Tirumalesh Chalamarla <tchalamarla@...ium.com>
> > 
> > Increase the standard cacheline size to avoid having locks in the same
> > cacheline.
> > 
> > Cavium's ThunderX core implements cache lines of 128 byte size. With
> > current granulare size of 64 bytes (L1_CACHE_SHIFT=6) two locks could
> > share the same cache line leading a performance degradation.
> > Increasing the size fixes that.
> 
> Do you have an example of that happening?

I did some 'poor man's kernel build all modules benchmarking' and
could not find significant performance improvements so far (second
part with the patch reverted):

 build-allmodules-4.2.0-01404-g5818d6e89783.log:real       7m10.490s
 build-allmodules-4.2.0-01404-g5818d6e89783.log:real       6m59.747s
 build-allmodules-4.2.0-01404-g5818d6e89783.log:real       6m59.264s
 build-allmodules-4.2.0-01404-g5818d6e89783.log:real       7m0.435s
 build-allmodules-4.2.0-01404-g5818d6e89783.log:real       6m59.569s
 build-allmodules-4.2.0-01404-g5818d6e89783.log:real       6m59.274s
 build-allmodules-4.2.0-01404-g5818d6e89783.log:real       7m0.507s
 build-allmodules-4.2.0-01404-g5818d6e89783.log:real       7m1.551s
 build-allmodules-4.2.0-01404-g5818d6e89783.log:real       6m59.073s
 build-allmodules-4.2.0-01404-g5818d6e89783.log:real       7m1.738s

 build-allmodules-4.2.0-01406-g638c69fddc40.log:real       7m10.644s
 build-allmodules-4.2.0-01406-g638c69fddc40.log:real       6m59.814s
 build-allmodules-4.2.0-01406-g638c69fddc40.log:real       7m0.315s
 build-allmodules-4.2.0-01406-g638c69fddc40.log:real       6m59.610s
 build-allmodules-4.2.0-01406-g638c69fddc40.log:real       6m59.885s
 build-allmodules-4.2.0-01406-g638c69fddc40.log:real       6m59.281s
 build-allmodules-4.2.0-01406-g638c69fddc40.log:real       7m0.869s
 build-allmodules-4.2.0-01406-g638c69fddc40.log:real       7m0.953s
 build-allmodules-4.2.0-01406-g638c69fddc40.log:real       7m0.787s
 build-allmodules-4.2.0-01406-g638c69fddc40.log:real       7m0.656s

I will check what kind of workloads this patch was written for.
Tirumalesh, any idea?

Thanks,

-Robert

> 
> > Increasing the size has no negative impact to cache invalidation on
> > systems with a smaller cache line. There is an impact on memory usage,
> > but that's not too important for arm64 use cases.
> 
> Do you have any before/after numbers to show the impact of this change
> on other supported SoCs?

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