linux-kernel - Re: [PATCH v4] f2fs: add sysfs nodes to get runtime compression stat

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Message-ID: <CACOAw_z9LUjx-5MRYnWOFnL9DzUkvKU1RVObRLwudZbpBxGywA@mail.gmail.com>
Date:   Sat, 13 Mar 2021 09:00:12 +0900
From:   Daeho Jeong <daeho43@...il.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, kernel-team@...roid.com,
        Daeho Jeong <daehojeong@...gle.com>
Subject: Re: [PATCH v4] f2fs: add sysfs nodes to get runtime compression stat

2021년 3월 12일 (금) 오후 11:45, Greg KH <gregkh@...uxfoundation.org>님이 작성:
>
> A: http://en.wikipedia.org/wiki/Top_post
> Q: Were do I find info about this thing called top-posting?
> A: Because it messes up the order in which people normally read text.
> Q: Why is top-posting such a bad thing?
> A: Top-posting.
> Q: What is the most annoying thing in e-mail?
>
> A: No.
> Q: Should I include quotations after my reply?
>
> http://daringfireball.net/2007/07/on_top
>

Thanks for letting me know this!

>
> On Fri, Mar 12, 2021 at 11:37:29PM +0900, Daeho Jeong wrote:
> > As you can see, if we're doing like the below.
> >
> > sbi->compr_written_block += blocks;
> >
> > Let's assume the initial value as 0.
> >
> > <thread A>                                             <thread B>
> > sbi->compr_written_block = 0;
> >
> > sbi->compr_written_block = 0;
> > +blocks(3);
> >                                                                + blocks(2);
> > sbi->compr_written_block = 3;
> >
> > sbi->compr_written_block = 2;
> >
> > Finally, we end up with 2, not 5.
> >
> > As more threads are participating it, we might miss more counting.
>
> Are you sure?  Isn't adding a number something that should happen in a
> "safe" way?
>
> And if you miss 2 blocks, who cares?  What is so critical about these
> things that you take the cache flush of 2 atomic writes just for a
> debugging statistic?
>
> Why not just take 1 lock for everything if it's so important to get
> these "correct"?
>
> What is the performance throughput degradation of adding 2 atomic writes
> to each time you write a block?
>
> But really, will you ever notice missing a few, even if that could be
> possible on your cpu (and I strongly doubt most modern cpus will miss
> this...)
>
> But this isn't my code, I just hate seeing atomic variables used for
> silly things like debugging stats when they do not seem to be really
> needed.  So if you want to keep them, go ahead, but realize that the
> number you are reading has nothing to do with being "atomic" at all.
>
> thanks,
>

I agree that missing number would be extremely few and the overhead of
updating the numbers would be quite bad.

Thanks for your valuable comments. :)

> greg k-h