Date: Sat, 23 Dec 2017 03:38:57 +0000 From: Colin Percival <cperciva@tarsnap.com> To: Mark Johnston <markj@FreeBSD.org> Cc: "freebsd-current@freebsd.org" <freebsd-current@freebsd.org> Subject: Re: [Request for review] Profiling the FreeBSD kernel boot Message-ID: <010001608173caa8-254b1eec-369a-4f21-af28-f0f68adc8506-000000@email.amazonses.com> In-Reply-To: <20171222170818.GA39071@raichu> References: <010001607d9c59d5-15b6b788-a7ea-4edf-96e4-1235dd1a5c26-000000@email.amazonses.com> <20171222170818.GA39071@raichu>
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On 12/22/17 09:08, Mark Johnston wrote: > On Fri, Dec 22, 2017 at 09:44:46AM +0000, Colin Percival wrote: >> For the past few months I've been working on code for profiling the FreeBSD >> "kernel boot", i.e., everything between when kernel code starts running and >> when we first enter userland as init(8). This is not trivial since it's >> impossible to use tools like dtrace to monitor things prior to when said >> tools are running. > > In the case of DTrace, this isn't quite true. We support so-called > boot-time DTrace on x86. The caveat is that we can only start tracing > after the SI_SUB_DTRACE_ANON sysinit has been executed. That sysinit > can't come earlier than SI_SUB_SMP, since it needs to be able to measure > TSC skew between CPUs in order to initialize DTrace's high-resolution > timer. Right. Also, even aside from details like measuring the TSC skew between CPUs, DTrace needs things like traps, memory allocation, and mutexes, none of which exist when we enter hammer_time (or any of the other MD startup code). What I meant is that it's impossible to use DTrace to monitor things which happened prior to when the DTrace *kernel bits* are initialized. > I don't think boot-time DTrace is quite what you want for this exercise, > but it does come in handy sometimes. Absolutely. And for a long time I considered trying to splice together a basic profiling mechanism for pre-DTrace-initialization with using DTrace from when it's ready onwards... but I decided that it would be easier at least to start with to simply use a single mechanism throughout. > In case it's of interest: to use boot-time DTrace, invoke dtrace(1) as > you normally would and add -A. Rather than starting to trace, dtrace(1) > will save a representation of the D script to a file which gets read by > the loader during the next boot. The results of the trace can be fetched > with "dtrace -a". For instance, to print the amount of time elapsed in > microseconds during each vprintf() call, along with a stack: [...] Thanks for the example! I think it's very likely that I'll make use of boot-time DTrace for tracking down some of the performance warts I've found -- the ones which happen after DTrace is initialized, that is. -- Colin Percival Security Officer Emeritus, FreeBSD | The power to serve Founder, Tarsnap | www.tarsnap.com | Online backups for the truly paranoid
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