From patchwork Fri May 28 17:45:29 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Ajit Khaparde X-Patchwork-Id: 93547 X-Patchwork-Delegate: thomas@monjalon.net Return-Path: X-Original-To: patchwork@inbox.dpdk.org Delivered-To: patchwork@inbox.dpdk.org Received: from mails.dpdk.org (mails.dpdk.org [217.70.189.124]) by inbox.dpdk.org (Postfix) with ESMTP id 7A0FBA0547; Fri, 28 May 2021 19:45:37 +0200 (CEST) Received: from [217.70.189.124] (localhost [127.0.0.1]) by mails.dpdk.org (Postfix) with ESMTP id B329D40687; Fri, 28 May 2021 19:45:36 +0200 (CEST) Received: from mail-pl1-f176.google.com (mail-pl1-f176.google.com [209.85.214.176]) by mails.dpdk.org (Postfix) with ESMTP id 9020D40040 for ; Fri, 28 May 2021 19:45:35 +0200 (CEST) Received: by mail-pl1-f176.google.com with SMTP id t21so1982066plo.2 for ; Fri, 28 May 2021 10:45:35 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=broadcom.com; s=google; h=from:to:cc:subject:date:message-id:mime-version; bh=+c9e0a09OE3k9YW6WkhVKNZa/ZJK3fW9h00UxcZJ2a0=; b=QjPEEHE6817ri3Cs8ob5v/v/mS2ZAg4pvonXy63MDUJ+4q+R/RuBNT8smzMDGOh29k HT0IB1Umab7dvHXreu1nH43gfth7/KsoxB1CoDYnoP38SI3NXaKcB0MyLe4jtCiU0P0j XQJ926zAr+xtRNxqBhdDmrh6n3mMYL8QYGFUc= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:cc:subject:date:message-id:mime-version; bh=+c9e0a09OE3k9YW6WkhVKNZa/ZJK3fW9h00UxcZJ2a0=; b=VR0Tn7T9UMvqtKBG2Nz8gKXt6NQUM1vEe8GjXbh2OVH0FK4qNpuE9SC1kjhfq4Vytt ehBdhxRBGjyEKiofwtgykVMFPB8tJrDphc/DUyNV4tiwS9OLMFL/cv9DQZf1Y6jFLztU mn9qZJujpnVxz3/F33qSpT6wuL3s9aIzJ+o1pkNYWCIkhCobiw24UCYUQTnBWVOMa9hM 0FY/Upvt3Yk+qpzkYky2MZr2r/2dpmKW08cf61REA92B5SFEkqic0vnEQZmnVUB+/ull 1Vowy5keNIkKAS+kxCDyNu2B5Z2PZDi7u92Yl5d5sg2XJw111mNto87vwY3tr9uUGxjw uQzw== X-Gm-Message-State: AOAM53182uG3FgixgerWu1SJaK+3XzwzLdBeQmrbSbt+Gpn205e7YJbq pfSXUzPYtn65GXJl9TDSNhrOeQqmYyI0LgV+p5wdpOigRUTO+T5WncYWHM6FP3nvzBxYW7Yz/56 /ZMCIDxiq1HWTYjKtVZJdzFd1GFAzlTfwlGyC/YDpB2nfwBmgfrYFZl4CTOtHb1I= X-Google-Smtp-Source: ABdhPJyRWfg1oEX2IEXEr2HKpz/9iH5vdCAMjnOV+6f9AL7/O14sd3h5p9UiS6lGmkY2/GQiaoC7vQ== X-Received: by 2002:a17:90a:a08c:: with SMTP id r12mr5626255pjp.204.1622223934202; Fri, 28 May 2021 10:45:34 -0700 (PDT) Received: from localhost.localdomain (ip68-5-71-166.oc.oc.cox.net. [68.5.71.166]) by smtp.gmail.com with ESMTPSA id y22sm4889273pjj.34.2021.05.28.10.45.32 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Fri, 28 May 2021 10:45:33 -0700 (PDT) From: Ajit Khaparde To: dev@dpdk.org Cc: stable@dpdk.org, bruce.richardson@intel.com, xiaoyun.li@intel.com, ferruh.yigit@intel.com, andrew.rybchenko@oktetlabs.ru Date: Fri, 28 May 2021 10:45:29 -0700 Message-Id: <20210528174529.84144-1-ajit.khaparde@broadcom.com> X-Mailer: git-send-email 2.21.1 (Apple Git-122.3) MIME-Version: 1.0 X-Content-Filtered-By: Mailman/MimeDel 2.1.29 Subject: [dpdk-dev] [PATCH] doc: fix default burst size 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 Sender: "dev" Default burst size in testpmd has been changed from 16 to 32 for some time now. But the documentation had not been updated. Fixes: 836853d3d4cf7 ("app/testpmd: increase default burst size to 32") Cc: stable@dpdk.org Cc: bruce.richardson@intel.com Cc: xiaoyun.li@intel.com Cc: ferruh.yigit@intel.com Cc: andrew.rybchenko@oktetlabs.ru Signed-off-by: Ajit Khaparde --- doc/guides/prog_guide/writing_efficient_code.rst | 15 ++++++++------- 1 file changed, 8 insertions(+), 7 deletions(-) diff --git a/doc/guides/prog_guide/writing_efficient_code.rst b/doc/guides/prog_guide/writing_efficient_code.rst index 7baeaae431..a61e8320ae 100644 --- a/doc/guides/prog_guide/writing_efficient_code.rst +++ b/doc/guides/prog_guide/writing_efficient_code.rst @@ -143,20 +143,21 @@ In order to achieve higher throughput, the DPDK attempts to aggregate the cost of processing each packet individually by processing packets in bursts. Using the testpmd application as an example, -the burst size can be set on the command line to a value of 16 (also the default value). -This allows the application to request 16 packets at a time from the PMD. +the burst size can be set on the command line to a value of 32 (also the default value). +This allows the application to request 32 packets at a time from the PMD. The testpmd application then immediately attempts to transmit all the packets that were received, -in this case, all 16 packets. +in this case, all 32 packets. The packets are not transmitted until the tail pointer is updated on the corresponding TX queue of the network port. This behavior is desirable when tuning for high throughput because -the cost of tail pointer updates to both the RX and TX queues can be spread across 16 packets, +the cost of tail pointer updates to both the RX and TX queues can be spread +across 32 packets, effectively hiding the relatively slow MMIO cost of writing to the PCIe* device. However, this is not very desirable when tuning for low latency because -the first packet that was received must also wait for another 15 packets to be received. -It cannot be transmitted until the other 15 packets have also been processed because +the first packet that was received must also wait for another 31 packets to be received. +It cannot be transmitted until the other 31 packets have also been processed because the NIC will not know to transmit the packets until the TX tail pointer has been updated, -which is not done until all 16 packets have been processed for transmission. +which is not done until all 32 packets have been processed for transmission. To consistently achieve low latency, even under heavy system load, the application developer should avoid processing packets in bunches.