Re: [PATCH] csum-file: flush less often

From: Jeff King <peff@peff.net>
Date: Mar 26, 2021, 03:16 UTC
Message-ID: <YF1SJRhb8hz7NP0/@coredump.intra.peff.net>
In-Reply-To: <xmqq4kgzvzf6.fsf@gitster.g>

On Thu, Mar 25, 2021 at 11:52:29AM -0700, Junio C Hamano wrote:

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> Junio C Hamano <gitster@pobox.com> writes:
> 
> >> So, I'm of two minds here:
> >>
> >>  1. This is embarassing. I wasted everyone's time for nothing. I can retract
> >>     this patch.
> >>
> >>  2. This is embarassing. I overstated the problem here. But we might be able
> >>     to eke out a tiny performance boost here.
> >>
> >> I'm open to either. I think we should default to dropping this patch unless
> >> someone thinks the rewrite above is a better organization of the logic. (I
> >> can then send a v2 including that version and an updated commit message.)
> >
> > 3. The current code around "if (nr == sizeof(f->buffer))" might be a
> >    bit too clever for readers who try to understand what is going
> >    on, and the whole "while" loop may deserve a comment based on
> >    what you wrote before your replacement implementation.

Yes, my first thought on reading Stolee's post-image was: wait, how do we know when data needed flushed from the buffer? But that is not new in his patch. It is confusing before and after. :)

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> Having said all that, comparing the original and the version updated
> with your "flush less often" patch, I find the latter quite easier
> to read, so as long as the update does not give us 1% slowdown, it
> may be worth adopting for the readability improvement alone.
> 
> Of course, if we were to go that route, the sales pitch in the log
> message needs to be updated.

Yeah, I am OK with either version, as long as it is justified correctly in the commit message. IMHO the big difference is that the original is using local data/offset variables in order to provide a layer of indirection when we get to the hash+flush code. And Stolee's patch is calling the same code in the two places instead.

It's quite possible that gives the compiler slightly more opportunity to micro-optimize (which doesn't matter if you are feeding big blocks, but may if you are feeding 4 bytes at a time as in the midx code; though in that case it is entirely possible that the caller allocating a single array, writing it, and then feeding it to hashwrite() would be faster still, though a little more cumbersome).

-Peff