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[
    {
        "id": 97818,
        "web_url": "http://patches.dpdk.org/comment/97818/",
        "msgid": "<20190630132958.6d2250da@hermes.lan>",
        "list_archive_url": "https://inbox.dpdk.org/dev/20190630132958.6d2250da@hermes.lan",
        "date": "2019-06-30T20:29:58",
        "subject": "Re: [dpdk-dev] [RFC 0/5] use WFE for locks and ring on aarch64",
        "submitter": {
            "id": 27,
            "url": "http://patches.dpdk.org/api/people/27/",
            "name": "Stephen Hemminger",
            "email": "stephen@networkplumber.org"
        },
        "content": "On Mon,  1 Jul 2019 00:21:11 +0800\nGavin Hu <gavin.hu@arm.com> wrote:\n\n> DPDK has multiple use cases where the core repeatedly polls a location in\n> memory. This polling results in many cache and memory transactions.\n> \n> Arm architecture provides WFE (Wait For Event) instruction, which allows\n> the cpu core to enter a low power state until woken up by the update to the\n> memory location being polled. Thus reducing the cache and memory\n> transactions.\n> \n> x86 has the PAUSE hint instruction to reduce such overhead.\n> \n> The rte_wait_until_equal_xxx APIs abstract the functionality of 'polling\n> for a memory location to become equal to a given value'.\n> \n> For non-Arm platforms, these APIs are just wrappers around do-while loop\n> with rte_pause, so there are no performance differences.\n> \n> For Arm platforms, use of WFE can be configured using CONFIG_RTE_USE_WFE\n> option. It is disabled by default.\n> \n> Currently, use of WFE is supported only for aarch64 platforms. armv7\n> platforms do support the WFE instruction, but they require explicit wake up\n> events(sev) and are less performannt.\n> \n> Testing shows that, performance varies across different platforms, with\n> some showing degradation.\n> \n> CONFIG_RTE_USE_WFE should be enabled depending on the performance on the\n> target platforms.\n\nHow does this work if process is preempted?",
        "headers": {
            "To": "Gavin Hu <gavin.hu@arm.com>",
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            "From": "Stephen Hemminger <stephen@networkplumber.org>",
            "Message-ID": "<20190630132958.6d2250da@hermes.lan>",
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            "References": "<1561911676-37718-1-git-send-email-gavin.hu@arm.com>",
            "In-Reply-To": "<1561911676-37718-1-git-send-email-gavin.hu@arm.com>",
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            "Subject": "Re: [dpdk-dev] [RFC 0/5] use WFE for locks and ring on aarch64",
            "Date": "Sun, 30 Jun 2019 13:29:58 -0700",
            "Precedence": "list",
            "X-Mailman-Version": "2.1.15",
            "Cc": "dev@dpdk.org, thomas@monjalon.net, jerinj@marvell.com,\n\themant.agrawal@nxp.com, bruce.richardson@intel.com,\n\tchaozhu@linux.vnet.ibm.com, Honnappa.Nagarahalli@arm.com, nd@arm.com"
        }
    },
    {
        "id": 97848,
        "web_url": "http://patches.dpdk.org/comment/97848/",
        "msgid": "<VI1PR08MB3167E6F95685A13D618BE8F48FF90@VI1PR08MB3167.eurprd08.prod.outlook.com>",
        "list_archive_url": "https://inbox.dpdk.org/dev/VI1PR08MB3167E6F95685A13D618BE8F48FF90@VI1PR08MB3167.eurprd08.prod.outlook.com",
        "date": "2019-07-01T09:12:47",
        "subject": "Re: [dpdk-dev] [RFC 0/5] use WFE for locks and ring on aarch64",
        "submitter": {
            "id": 1018,
            "url": "http://patches.dpdk.org/api/people/1018/",
            "name": "Gavin Hu",
            "email": "gavin.hu@arm.com"
        },
        "content": "Hi Stephen,\n\n> -----Original Message-----\n> From: Stephen Hemminger <stephen@networkplumber.org>\n> Sent: Monday, July 1, 2019 4:30 AM\n> To: Gavin Hu (Arm Technology China) <Gavin.Hu@arm.com>\n> Cc: dev@dpdk.org; thomas@monjalon.net; jerinj@marvell.com;\n> hemant.agrawal@nxp.com; bruce.richardson@intel.com;\n> chaozhu@linux.vnet.ibm.com; Honnappa Nagarahalli\n> <Honnappa.Nagarahalli@arm.com>; nd <nd@arm.com>\n> Subject: Re: [dpdk-dev] [RFC 0/5] use WFE for locks and ring on aarch64\n> \n> On Mon,  1 Jul 2019 00:21:11 +0800\n> Gavin Hu <gavin.hu@arm.com> wrote:\n> \n> > DPDK has multiple use cases where the core repeatedly polls a location in\n> > memory. This polling results in many cache and memory transactions.\n> >\n> > Arm architecture provides WFE (Wait For Event) instruction, which allows\n> > the cpu core to enter a low power state until woken up by the update to the\n> > memory location being polled. Thus reducing the cache and memory\n> > transactions.\n> >\n> > x86 has the PAUSE hint instruction to reduce such overhead.\n> >\n> > The rte_wait_until_equal_xxx APIs abstract the functionality of 'polling\n> > for a memory location to become equal to a given value'.\n> >\n> > For non-Arm platforms, these APIs are just wrappers around do-while loop\n> > with rte_pause, so there are no performance differences.\n> >\n> > For Arm platforms, use of WFE can be configured using\n> CONFIG_RTE_USE_WFE\n> > option. It is disabled by default.\n> >\n> > Currently, use of WFE is supported only for aarch64 platforms. armv7\n> > platforms do support the WFE instruction, but they require explicit wake up\n> > events(sev) and are less performannt.\n> >\n> > Testing shows that, performance varies across different platforms, with\n> > some showing degradation.\n> >\n> > CONFIG_RTE_USE_WFE should be enabled depending on the performance\n> on the\n> > target platforms.\n> \n> How does this work if process is preempted?\nWFE won't prevent pre-emption from the kernel as that is down to a timer/re-scheduling interrupt.\nSoftware using the WFE mechanism must tolerate spurious wake-up events, including timer/re-scheduling interrupts, so a re-check of the condition upon exit of WFE is needed to be in place(this is already included in the patch)",
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            "Date": "Mon, 1 Jul 2019 09:12:47 +0000",
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            "Subject": "Re: [dpdk-dev] [RFC 0/5] use WFE for locks and ring on aarch64",
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            "From": "\"Gavin Hu (Arm Technology China)\" <Gavin.Hu@arm.com>",
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            "Thread-Topic": "[dpdk-dev] [RFC 0/5] use WFE for locks and ring on aarch64",
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