linux-kernel - Re: arm64 + ARM64_64K

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Message-ID: <20181108120040.pzupzyqnsxeuv5ne@linutronix.de>
Date:   Thu, 8 Nov 2018 13:00:40 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Grygorii Strashko <grygorii.strashko@...com>
Cc:     linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        Linux ARM Mailing List <linux-arm-kernel@...ts.infradead.org>
Subject: Re: arm64 + ARM64_64K_PAGES=y

On 2018-11-06 15:34:55 [-0600], Grygorii Strashko wrote:
> Hi All,
Hi,

> Do anybody tried to use ARM64 RT with 76K pages enabled?

75 would be an off by one but this :)

> My attempt shows that enabling  CONFIG_ARM64_64K_PAGES=y increases latencies by ~30%
> 
> cyclictest -n -m -Sp98 -q -D2m with CONFIG_ARM64_64K_PAGES=y
> 
> 
> T: 0 (  772) P:98 I:1000 C: 120000 Min:      7 Act:   13 Avg:   10 Max:      85
> T: 1 (  773) P:98 I:1500 C:  79998 Min:      7 Act:   13 Avg:   10 Max:      71
> T: 2 (  774) P:98 I:2000 C:  59997 Min:      7 Act:   11 Avg:   11 Max:      64
> T: 3 (  775) P:98 I:2500 C:  47996 Min:      7 Act:   14 Avg:   12 Max:      66
> 
> 
> cyclictest -n -m -Sp98 -q -D2m with CONFIG_ARM64_64K_PAGES=n
> 
> 
> T: 0 (  697) P:98 I:1000 C: 120000 Min:      7 Act:   10 Avg:    9 Max:      38
> T: 1 (  698) P:98 I:1500 C:  79987 Min:      7 Act:   10 Avg:   10 Max:      32
> T: 2 (  699) P:98 I:2000 C:  59981 Min:      7 Act:   14 Avg:   11 Max:      46
> T: 3 (  700) P:98 I:2500 C:  47977 Min:      6 Act:   11 Avg:   10 Max:      45

So this is an idle system?
The Kconfig help says "faster TLB lookup". Interesting.
Are the 16k pages in between (latency wise) by any chance?

> -- 
> regards,
> -grygorii

Sebastian