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[204.195.112.199]) by smtp.gmail.com with ESMTPSA id n22-20020a17090a161600b001ecd48b80a2sm11879229pja.5.2022.06.28.08.19.20 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 28 Jun 2022 08:19:20 -0700 (PDT) Date: Tue, 28 Jun 2022 08:19:18 -0700 From: Stephen Hemminger <stephen@networkplumber.org> To: Carsten Andrich <carsten.andrich@tu-ilmenau.de> Cc: users@dpdk.org Subject: Re: Large interruptions for EAL thread running on isol core Message-ID: <20220628081918.0fc0f31c@hermes.local> In-Reply-To: <0906411a-90ad-5651-0153-e34a29027f66@tu-ilmenau.de> References: <CAO8pfF=yo37Ng5QAcopL=6LgKMHr4+=pPcONPATyS+OD0GLgfg@mail.gmail.com> <9a95a560-c7c6-d4fa-1041-836fa400f497@tu-ilmenau.de> <20220624080124.244a01a9@hermes.local> <ef44e436-8e3b-7083-10d8-0dc1602581c8@tu-ilmenau.de> <20220624094243.232f955e@hermes.local> <0906411a-90ad-5651-0153-e34a29027f66@tu-ilmenau.de> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-BeenThere: users@dpdk.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: DPDK usage discussions <users.dpdk.org> List-Unsubscribe: <https://mails.dpdk.org/options/users>, <mailto:users-request@dpdk.org?subject=unsubscribe> List-Archive: <http://mails.dpdk.org/archives/users/> List-Post: <mailto:users@dpdk.org> List-Help: <mailto:users-request@dpdk.org?subject=help> List-Subscribe: <https://mails.dpdk.org/listinfo/users>, <mailto:users-request@dpdk.org?subject=subscribe> Errors-To: users-bounces@dpdk.org On Tue, 28 Jun 2022 09:25:50 +0200 Carsten Andrich <carsten.andrich@tu-ilmenau.de> wrote: > On 24.06.22 17:01, Stephen Hemminger wrote: > > On Thu, 23 Jun 2022 21:03:49 +0200 > > Carsten Andrich <carsten.andrich@tu-ilmenau.de> wrote: > > > >> 2. Use real-time priority (SCHED_FIFO w/ priority 99) for the DPDK > >> threads and > >> echo -1 > /proc/sys/kernel/sched_rt_runtime_us > >> to disable the runtime limit. With the runtime limit in place, the > >> SCHED_FIFO performance will be significantly worse than SCHED_OTHER. > > This can cause major issues if application is normal DPDK application (never does system calls). > > If an interrupt or other event happens on your isolated CPU, the work that it would > > do in soft irq is never performed. FIFO has higher priority than kernel threads. > > This can lead to mystery lockups from other applications (reads not completing, network timeouts, etc). > > Thanks for pointing that out. Do you know of any official kernel > documentation that could shed some light on that? I haven't had any > serious issues like the ones you list, but maybe I've been lucky. My > DPDK applications typically run on fairly minimal systems used > exclusively for DPDK tasks, which require minimal latency/jitter. Minor > side-effects from using SCHED_FIFO are tolerable in my case, if it > improves performance. Do some looking around and you will find good documentation like: https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux_for_real_time/7/html/tuning_guide/real_time_throttling This characteristic of real-time threads means that it is quite easy to write an application which monopolizes 100% of a given CPU. At first glance this sounds like it might be a good idea, but in reality it causes lots of headaches for the operating system. The OS is responsible for managing both system-wide and per-CPU resources and must periodically examine data structures describing these resources and perform housekeeping activities with them. If a core is monopolized by a SCHED_FIFO thread, it cannot perform the housekeeping tasks and eventually the entire system becomes unstable, potentially causing a crash.