Date: Wed, 7 Dec 2016 15:34:02 +0200 From: Alexander Motin <mav@FreeBSD.org> To: freebsd-fs@FreeBSD.org Cc: Alex Tutubalin <lexa@lexa.ru>, Andriy Gapon <avg@FreeBSD.org> Subject: Re: 11-STABLE vs 11.0-RELENG test Message-ID: <374b6d16-5cb4-9338-ec1d-65ad93ca29dc@FreeBSD.org> In-Reply-To: <8b4ba98d-03d3-f671-33b2-ed12d3b4fb7c@FreeBSD.org> References: <8b4ba98d-03d3-f671-33b2-ed12d3b4fb7c@FreeBSD.org>
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On 06.12.2016 22:26, Alexander Motin wrote: > I've reproduced this issue with quick test on my lab system configured > with 12-disk RAIDZ2 pool. I've measured write and read back (with and > without prefetch) speeds for pool recreated on different FreeBSD head > revisions: > r309625 r305456 r305330 r305322 > write 702 701 1115 1120 > read w/ pref 232 228 518 512 > read w/o pref 128 126 242 240 > > I suspect we could obtain the problem here: > > r305331 | mav | 2016-09-03 13:04:37 +0300 (сб, 03 сент. 2016) | 45 lines > > MFV r304155: 7090 zfs should improve allocation order and throttle > allocations Closer look shown me the cause. This code sorts I/Os on time, offset and memory address. But time on FreeBSD (to reduce overhead) returned with 1ms resolution, so it does not provide reliable ordering. Offset sorting used by this patch is broken by design, since io_offset field is always zero there, since it is used only for physical I/Os, not for logical. As result, I/Os are "sorted" on memory address, that in fact means complete randomization of all allocations within one millisecond, predictably killing read performance. Switching gethrtime() emulation from getnanouptime() to nanouptime() fixes the read performance, resulting: nanouptime() write 702 read w/ pref 845 read w/o pref 272 It would be good to make offset sorting really work there rather then just switching to high resolution time source, but that maybe quite invasive. Will look more. -- Alexander Motin
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