Subject: Re: [PATCH 2/3] strbuf.h API users: don't hardcode 8192, use STRBUF_HINT_SIZE

From: Junio C Hamano <[email-masked]>
Date: Wed, 07 Jul 2021 16:09:44 -0700

Junio C Hamano <[email-masked]> writes:

> Ævar Arnfjörð Bjarmason  <[email-masked]> writes:
>
>> Change a couple of users of strbuf_init() that pass a hint of 8192 to
>> pass STRBUF_HINT_SIZE instead.
>>
>> Both of these hardcoded occurrences pre-date the use of the strbuf
>> API. See 5242bcbb638 (Use strbuf API in cache-tree.c, 2007-09-06) and
>> af6eb82262e (Use strbuf API in apply, blame, commit-tree and diff,
>> 2007-09-06).
>>
>> In both cases the exact choice of 8192 is rather arbitrary, e.g. for
>> commit buffers I think 1024 or 2048 would probably be a better
>> default (this commit message is getting this commit close to the
>> former, but I daresay it's already way above the average for git
>> commits).
>
> Yes, they are arbitrary within the context of these callers.
>
> I do not think using STRBUF_HINT_SIZE macro in them is the right
> thing to do at all, as there is no reason to think that the best
> value for the write chunk sizes in these codepath has any linkage to
> the best value for the read chunk sizes used by strbuf_read() at
> all.  When benchmarking reveals that the best default size for
> strbuf_read() is 16k, you'd update STRBUF_HINT_SIZE to 16k, but how
> do you tell that it also happens to be the best write buffer size
> for the cache-tree writeout codepath (answer: you don't)?

Having said all that, I wouldn't be so opposed to an approach that

 - declares that we need only one "default I/O buffer size";

 - declares that the appropriate size for it is 8192;

 - #define DEFAULT_IO_SIZE 8192;

 - does something like your [PATCH 1/3], but not limited to strbuf
   API, and

 - covers also the writeout codepath of cache-tree, etc. that uses
   hardcoded I/O buffer size.

The biggest trouble I had with the posted patches, especially the
[PATCH 2/3], was that I am quite sure that you wouldn't have used
STRBUF_HINT_SIZE in the cache-tree writeout codepath or commit-tree
codepath if they didn't use strbuf as a convenient way to get an
elastic buffer.  The more relevant commonality across codepaths that
use 8192 is that the constant is used for sizing the I/O buffer, and
I got an impression that the 3-patch series posted did an incomplete
job of touching some that happen to use strbuf.

An approach that concentrated on the "right" commonality, i.e. we
have hardcoded magic constants for I/O buffer sizing, would have
covered copy.c, diff-delta.c, http-backend.c etc. that do not use
strbuf API where they have hardcoded 8k constants.

Thanks.