Junio C Hamano <gitster@xxxxxxxxx> writes: > Ævar Arnfjörð Bjarmason <avarab@xxxxxxxxx> 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.
