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c1b5801bb5
For busy ARC situation when arc_size close to arc_c is desired. But then it is quite likely that aggsum_compare(&arc_size, arc_c) will need to flush per-CPU buckets to find exact comparison result. Doing that often in a hot path penalizes whole idea of aggsum usage there, since it replaces few simple atomic additions with dozens of lock acquisitions. Replacing aggsum_compare() with aggsum_upper_bound() in code increasing arc_p when ARC is growing (arc_size < arc_c) according to PMC profiles allows to save ~5% of CPU time in aggsum code during sequential write to 12 ZVOLs with 16KB block size on large dual-socket system. I suppose there some minor arc_p behavior change due to lower precision of the new code, but I don't think it is a big deal, since it should affect only very small window in time (aggsum buckets are flushed every second) and in ARC size (buckets are limited to 10 average ARC blocks per CPU). Reviewed-by: Chris Dunlop <chris@onthe.net.au> Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com> Reviewed-by: George Melikov <mail@gmelikov.ru> Reviewed-by: Allan Jude <allanjude@freebsd.org> Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Signed-off-by: Alexander Motin <mav@FreeBSD.org> Closes #8901 |
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