Date: Sat, 21 Mar 2015 09:31:02 -0700 From: Adrian Chadd <adrian@freebsd.org> To: John Baldwin <jhb@freebsd.org> Cc: Konstantin Belousov <kostikbel@gmail.com>, FreeBSD-STABLE Mailing List <freebsd-stable@freebsd.org>, John-Mark Gurney <jmg@funkthat.com>, Xin LI <d@delphij.net> Subject: Re: RELENG_10 performance regression (was Re: 35-40% performance drop releng9 vs releng10 openvpn Message-ID: <CAJ-VmonSAqgJLvYG4TRXdnEYJdct96qvZn2LNYyWCHKaFjuYSg@mail.gmail.com> In-Reply-To: <550D93C7.9080709@FreeBSD.org> References: <5506250A.2000506@sentex.net> <20150316132055.GQ32288@funkthat.com> <5509D6C6.4050204@sentex.net> <20150318211457.GL51048@funkthat.com> <550B6950.8060806@sentex.net> <550C5AAF.9060502@sentex.net> <550C8AEE.4090408@sentex.net> <550CB306.7030405@delphij.net> <20150321001559.GB2379@kib.kiev.ua> <550CBF80.6030809@sentex.net> <550D93C7.9080709@FreeBSD.org>
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On 21 March 2015 at 08:52, John Baldwin <jhb@freebsd.org> wrote: > On 3/20/15 8:46 PM, Mike Tancsa wrote: >> On 3/20/2015 8:15 PM, Konstantin Belousov wrote: >>>> >>>> For the purpose of devfs, does it make sense to bump timestamps like >>>> normal filesystems for each read/write operation? Looks like Mac OS X >>>> will bump timestamps for each operation but Debian don't. >>> >>> First question is, what timecounter hardware is used. I would accept >>> some slowdown from hardware like HPET, but it is indeed surprising >>> if caused by TSC. >>> >>> >> >> David Wolfskill suggested trying the problem commit with >> >> vfs.timestamp_precision=0 >> >> and it does indeed restore performance to what it was. The raw dtrace >> files are available and FlameGraphs can all be found at >> >> http://tancsa.com/time/ > > Do you know why you are using the HPET instead of TSC for timestamping? > Using the TSC can make a non-trivial performance difference since userland > can calculate timestamps without using system calls when it is used. > (That is not related to this case, but switching to the TSC in general is > preferable.) > > There are a few generations of Intel CPUs where you can't mix deeper sleep > states with the TSC as timecounter, but those CPUs are getting to be a bit > older at this point. What about various VMs? -adrian
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