linux-kernel - Re: Recent slowdown in single object builds

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Date:   Sat, 16 Nov 2019 09:57:06 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Recent slowdown in single object builds

On 11/16/19 12:17 AM, Masahiro Yamada wrote:
> On Sat, Nov 16, 2019 at 8:10 AM Jens Axboe <axboe@...nel.dk> wrote:
>>
>> Hi,
>>
>> I've noticed that current -git is a lot slower at doing single object
>> builds than earlier kernels. Here's an example, building the exact same
>> file on 5.2 and -git:
>>
>> $ time make fs/io_uring.o
>> real    0m5.953s
>> user    0m5.402s
>> sys     0m0.649s
>>
>> vs 5.2 based (with all the backports, identical file):
>>
>> $ time make fs/io_uring.o
>> real    0m3.218s
>> user    0m2.968s
>> sys     0m0.520s
>>
>> Any idea what's going on here? It's almost twice as slow, which is
>> problematic...
> 
> 
> 
> This is necessary cost
> to do single builds
> (394053f4a4b3e3eeeaa67b67fc886a9a75bd9e4d)
> but, it is much better in linux-next.

Very sad that it's now twice as slow as before, that's a real problem.
This isn't a marginal slowdown.

I've never had issues with single object builds before, and in fact
it's often useful to build stuff that's disabled as a single object.
Where's the report that led to this commit being necessary?

-- 
Jens Axboe