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CTRY:IE; LANG:en; SCL:1; SRV:; IPV:CAL; SFV:NSPM; H:64aa7808-outbound-1.mta.getcheckrecipient.com; PTR:ec2-63-35-35-123.eu-west-1.compute.amazonaws.com; CAT:NONE; SFS:(13230040)(376014)(82310400026)(35042699022)(36860700013)(1800799024); DIR:OUT; SFP:1101; X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 08 Oct 2024 15:09:21.6554 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: e5e408f0-6ea2-4bd0-6bb5-08dce7ab305c X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d; Ip=[63.35.35.123]; Helo=[64aa7808-outbound-1.mta.getcheckrecipient.com] X-MS-Exchange-CrossTenant-AuthSource: DU2PEPF0001E9C0.eurprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: GVXPR08MB10520 X-BeenThere: dev@dpdk.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org > 1. rte_ring_generic_pvt.h: > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D >=20 > pseudo-c-code // related armv= 8 instructions > -------------------- ----= ---------------------------------- > head.load() // ldr [head= ] > rte_smp_rmb() // dmb ishld > opposite_tail.load() // ldr [opposite_= tail] > ... > rte_atomic32_cmpset(head, ...) // ldrex[head];... stlex[head= ] >=20 >=20 > 2. rte_ring_c11_pvt.h > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D >=20 > pseudo-c-code // related arm= v8 instructions > -------------------- ----= ---------------------------------- > head.atomic_load(relaxed) // ldr[head] > atomic_thread_fence(acquire) // dmb ish > opposite_tail.atomic_load(acquire) // lda[opposite_tail] > ... > head.atomic_cas(..., relaxed) // ldrex[haed]; ... str= ex[head] >=20 >=20 > 3. rte_ring_hts_elem_pvt.h > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D >=20 > pseudo-c-code // related arm= v8 instructions > -------------------- ----= ---------------------------------- > head.atomic_load(acquire) // lda [head] > opposite_tail.load() // ldr [opposite_= tail] > ... > head.atomic_cas(..., acquire) // ldaex[head]; ... stre= x[head] >=20 > The questions that arose from these observations: > a) are all 3 approaches equivalent in terms of functionality? Different, lda (Load with acquire semantics) and ldr (load) are different.= =20 > b) if yes, is there any difference in terms of performance between: > "ldr; dmb; ldr;" vs "lda; ldr;" > ? dmb is a full barrier, performance is poor. I would assume (haven't measured) ldr; dmb; ldr to be less performant than = lda;ldr; > c) Comapring at 1) and 2) above, combination of > ldr [head]; dmb; lda [opposite_tail]: > looks like an overkill to me. Wouldn't just: > ldr [head]; dmb; ldr[opposite_tail]; > be sufficient here? lda [opposite_tail]: synchronizes with stlr in tail update that happens aft= er array update. So, it cannot be changed to ldr.=20 lda can be replaced with ldapr (LDA with release consistency - processor co= nsistency)=20 which is more performant as lda is allowed to rise above stlr. Can be done = with -mcpu=3D+rcpc --wathsala