RFR(XS): 8151030: PPC64: AllocatePrefetchStyle=4 is out of range

[Michihiro Horie]

Hi Martin,

I started measuring SPECjbb2015 to see the performance impact on P9. Also,
I'm preparing same measurement on P8.

Best regards,
Michihiro


 ----- Original message -----
 From: "Doerr, Martin" <martin.doerr at sap.com>
 To: "'hotspot-compiler-dev at openjdk.java.net'"
 <hotspot-compiler-dev at openjdk.java.net>
 Cc: Michihiro Horie <HORIE at jp.ibm.com>, "cjashfor at linux.ibm.com"
 <cjashfor at linux.ibm.com>, "ppc-aix-port-dev at openjdk.java.net"
 <ppc-aix-port-dev at openjdk.java.net>, Gustavo Romero
 <gromero at linux.vnet.ibm.com>, "joserz at linux.ibm.com"
 <joserz at linux.ibm.com>
 Subject: [EXTERNAL] RFR(XS): 8151030: PPC64: AllocatePrefetchStyle=4 is
 out of range
 Date: Tue, Apr 14, 2020 11:07 PM

 Hi,

 I’d like to resolve a very old PPC64 issue:
 https://bugs.openjdk.java.net/browse/JDK-8151030

 There’s code for AllocatePrefetchStyle=4 which is not an accepted option.
 It was used for a special experimental prefetch mode using dcbz
 instructions to combine prefetching and zeroing in the TLABs.
 However, this code was never contributed and there are no plans to work on
 it. So I’d like to simply remove this small part of it.

 In addition to that, AllocatePrefetchLines is currently set to 3 by
 default which doesn’t make sense to me. PPC64 has an automatic prefetch
 engine and executing several prefetch instructions for succeeding cache
 lines doesn’t seem to be beneficial at all.
 So I’m setting it to 1 by default. I couldn’t observe regressions on
 Power7, Power8 and Power9.

 Webrev:
 http://cr.openjdk.java.net/~mdoerr/8151030_ppc_prefetch/webrev.00/

 Please review.

 If somebody from IBM would like to check performance impact of changing
 the AllocatePrefetchLines + Distance, I’ll be glad to receive feedback.

 Best regards,
 Martin