linux-kernel - Re: [PATCH 04/14] init: deps: order network interfaces by link order

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Date:	Thu, 22 Oct 2015 08:47:33 +0200
From:	Alexander Holler <holler@...oftware.de>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Russell King <linux@....linux.org.uk>,
	Grant Likely <grant.likely@...aro.org>
Subject: Re: [PATCH 04/14] init: deps: order network interfaces by link order

Am 19.10.2015 um 13:31 schrieb Alexander Holler:
> Am 19.10.2015 um 12:57 schrieb Alexander Holler:
>> Am 18.10.2015 um 12:11 schrieb Alexander Holler:
>>> Am 18.10.2015 um 07:59 schrieb Greg Kroah-Hartman:
>>>> On Sun, Oct 18, 2015 at 07:20:34AM +0200, Alexander Holler wrote:
>>>>> Am 18.10.2015 um 07:14 schrieb Greg Kroah-Hartman:
>>>>>> On Sun, Oct 18, 2015 at 06:59:22AM +0200, Alexander Holler wrote:
>>>>>>> Am 17.10.2015 um 21:36 schrieb Greg Kroah-Hartman:
>>>>>>>
>>>>>>>> Again, parallelizing does not solve anything, and causes more
>>>>>>>> problems
>>>>>>>> _and_ makes things take longer.  Try it, we have done it in the
>>>>>>>> past and
>>>>>>>> proven this, it's pretty easy to test :)
>>>>>>>
>>>>>>> Just because I'm curious, may I ask how I would test that in the
>>>>>>> easy way
>>>>>>> you have in mind? I've just posted the results of my tests (the
>>>>>>> patch
>>>>>>> series) but I wonder what you do have in mind.
>>>>>>
>>>>>> Use the tool, scripts/bootgraph.pl to create a boot graph of your
>>>>>> boot
>>>>>> sequence.  That should show you the drivers, or other areas, that are
>>>>>> causing your boot to be "slow".
>>>>>
>>>>> So I've misunderstood you. I've read your paragraph as that it's
>>>>> easy to
>>>>> test parallelizing.
>>>>
>>>> Ah, ok, if you want to parallelize everything, add some logic in the
>>>> driver core where the probe() callback is made to spin that off into a
>>>> new thread for every call, and when it's done, clean up the thread.
>>>> That's what I did many years ago to try this all out, if you dig in the
>>>> lkml archives there's probably a patch somewhere that you can base the
>>>> work off of to test it yourself.
>>>
>>> Hmm, I don't think I will do that because that means to setup a new
>>> thread for every call. And it doesn't need much imagination (or
>>> experience) that this introduces quite some overhead.
>>>
>>> But maybe it makes sense to try out what I'm doing in my patches,
>>> starting multiple threads once and then just giving them some work. Will
>>
>> After a having second thought on your simple approach to parallelize
>> stuff, I have to say that it just can't work because just starting a
>> thread for every probe() totally ignores possible dependencies.
>> Regardless if using one thread per probe() call or if feeding probe()
>> calls to just a few threads.
>>
>> Maybe that's why previous attempts to parallelize stuff failed. But
>> that's just an assumption as I'm unaware of these previous attempts.
>
> Or to describe it more verbose, if DEBUG is turned on in
> init/dependencies.c (using my patches), it spits out a summary of groups
> with initcalls (probe() calls) which are independent from each other and
> therfore can be called in parallel. E.g. one of my systems this looks so:
>
> [    0.288229] init: vertices: 429 edges 204 count 170
> [    0.288295] init: group 0 length 66 (start 0)
> [    0.288329] init: group 1 length 33 (start 66)
> [    0.288364] init: group 2 length 13 (start 99)
> [    0.288398] init: group 3 length 7 (start 112)
> [    0.288432] init: group 4 length 9 (start 119)
> [    0.288466] init: group 5 length 8 (start 128)
> [    0.288500] init: group 6 length 11 (start 136)
> [    0.288534] init: group 7 length 6 (start 147)
> [    0.288569] init: group 8 length 4 (start 153)
> [    0.288603] init: group 9 length 8 (start 157)
> [    0.288637] init: group 10 length 3 (start 165)
> [    0.288671] init: group 11 length 2 (start 168)
> [    0.288705] init: using 4 threads to call annotated initcalls
>
> That means the first group contains 66 initcalls which are called using
> 4 threads, and, after those have finished, the second group with 33
> initcalls will be called in parallel (using the same 4 threads).

BTW. That also means that, for the above example, the worst case would 
mean an error rate of 61% if those 170 (annotated) initcalls would be 
started in parallel while ignoring dependencies.

But that's just meant as an (hopefully) interesting number when looking 
at the above numbers a bit different.

(I've understood that the patches aren't wanted.)

>> Regards,
>>
>> Alexander Holler

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