Re: [PATCH] read-cache: make the index write buffer size 128K

From: Chris Torek <chris.torek@gmail.com>
Date: Feb 25, 2021, 06:58 UTC
Message-ID: <CAPx1GvdA1prtO+y-bJ7yu8oZP6Lp9mHQ5gv-fXvS193NFospkA@mail.gmail.com>
In-Reply-To: <xmqqmtvswvp7.fsf@gitster.g>

Show 12 quoted lines

> Neeraj Singh <nksingh85@gmail.com> writes:
> > I collected data from an experiment with different buffer sizes on Windows on my
> > 3.6Ghz Xeon W-2133 machine:
> > https://docs.google.com/spreadsheets/d/1Bu6pjp53NPDK6AKQI_cry-hgxEqlicv27dptoXZYnwc/edit?usp=sharing
> >
> > The timing is pretty much in the noise after we pass 32K.  So I think
> > 8K is too small, but
> > given the flatness of the curve we can feel good about any value above
> > 32K from a performance
> > perspective.  I still think 128K is a decent number that won't likely
> > need to be changed for
> > some time.

Linux/BSD/etc `stat` system calls report st_blksize values to tell user code the optimal size for read and write calls. Does Windows have one? (It's not POSIX but is XSI.)

(How *well* the OS reports `st_blksize` is another question entirely, but at least if the report says, say, 128k, and that's wrong, that's no longer Git's fault. :-) )

On Wed, Feb 24, 2021 at 10:46 PM Junio C Hamano <gitster@pobox.com> wrote:

Show 9 quoted lines

> Thanks for a supporting graph.
>
> I can very well imagine that it would have been tempting to instead
> say "after we pass 128k" while explaining exactly the same graph,
> and doing so would have given a more coherent argument to support
> the choice of 128k the patch made.  You knew that a "then perhaps we
> can reclaim 96k by sizing the buffer down a bit?" would become a
> reasonable response, but you still chose to be honest, which I kinda
> like ;-)

128K is correct for ZFS; 64K is typically correct for UFS2; 8K is the old UFS1 size. Anything under that has been too small for a long time. :-)

Chris