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A mirror of the official Linux kernel repository just in case
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Inter-Processor-Interrupt(IPI) is needed when a page is unmapped and the process' mm_cpumask() shows the process has ever run on other CPUs. page migration, page reclaim all need IPIs. The number of IPI needed to send to different CPUs is especially large for multi-threaded workload since mm_cpumask() is per process. For smp_call_function_many(), whenever a CPU queues a CSD to a target CPU, it will send an IPI to let the target CPU to handle the work. This isn't necessary - we need only send IPI when queueing a CSD to an empty call_single_queue. The reason: flush_smp_call_function_queue() that is called upon a CPU receiving an IPI will empty the queue and then handle all of the CSDs there. So if the target CPU's call_single_queue is not empty, we know that: i. An IPI for the target CPU has already been sent by 'previous queuers'; ii. flush_smp_call_function_queue() hasn't emptied that CPU's queue yet. Thus, it's safe for us to just queue our CSD there without sending an addtional IPI. And for the 'previous queuers', we can limit it to the first queuer. To demonstrate the effect of this patch, a multi-thread workload that spawns 80 threads to equally consume 100G memory is used. This is tested on a 2 node broadwell-EP which has 44cores/88threads and 32G memory. So after 32G memory is used up, page reclaiming starts to happen a lot. With this patch, IPI number dropped 88% and throughput increased about 15% for the above workload. Signed-off-by: Aaron Lu <aaron.lu@intel.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Dave Hansen <dave.hansen@intel.com> Cc: Huang Ying <ying.huang@intel.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Tim Chen <tim.c.chen@linux.intel.com> Link: http://lkml.kernel.org/r/20170519075331.GE2084@aaronlu.sh.intel.com Signed-off-by: Ingo Molnar <mingo@kernel.org> |
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.