Commits on Jun 15, 2023

1. Match concurrency to available CPU bandwidth …

   This change allows ninja to throttle number of parallel tasks based
   on feedback from Linux kernel's PSI (Pressure Stall Information)
   interfacts.  It extends "-l" parameter to accept negative values;
   "-l-NN" means that ninja should limit concurrency when processes
   in current cgroup spend more than NN% of their time stalled on CPU.
   
   E.g., running "ninja -j100 -l-10" on a 32-core machine will quickly
   settle on parallelism of 32-34.
   
   This option is designed to make ninja use all CPU bandwidth available
   to a cgroup-based container, while not starting excessive number of
   processes, which could eat up all RAM.
   
   The motivation for this feature is to automatically reduce
   parallelism when the system is about to run out of RAM.
   If the system has swap enabled, "ninja -l-10" will dance with
   parallelism on the edge of just using a bit of swap.  As soon as
   a process starts swapping, CPU "stalled" cycles increase, and
   parallelism is reduced.  The same argument works when a process is
   waiting for its turn to use IO and/or network.

   

   maxim-kuvyrkov committed Jun 15, 2023
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