# Re-instate index file write optimization 1 messages from 2005-10-01 to 2005-10-01. Participants: Linus Torvalds. Thread: https://gitlist.dev/t/1992 ## Linus Torvalds, 2005-10-01 20:39 Subject: Re-instate index file write optimization Message-ID: URL: https://gitlist.dev/e/Pine.LNX.4.64.0510011331570.3378%40g5.osdl.org ``` This makes "git-update-index" avoid the new index file write if it didn't make any changes to the index. It still doesn't make things like "git status" be read-only operations in general, but if the index file doesn't need refreshing, it now will at least avoid making unnecessary changes. Signed-off-by: Linus Torvalds --- This only makes sense after applying my previous patch, since otherwise a "--refresh" will always end up marking the index dirty anyway. Also, while I tried to verify that we always mark the index dirty when we change it, I can't guarantee it. In particular, since we now map the index file with PROT_READ | PROT_WRITE, somebody can modify the index entries in place without getting a SIGSEGV. Originally you couldn't do the modify-in-place, and used to be that you could depend on the active_cache[] being unmodified only by actually assigning to the pointer array itself. I don't think git-update-index writes to the entries directly, though (git-read-tree does, but it doesn't do the optimization). diff --git a/update-index.c b/update-index.c --- a/update-index.c +++ b/update-index.c @@ -391,9 +391,11 @@ int main(int argc, const char **argv) update_one(buf.buf, prefix, prefix_length); } } - if (write_cache(newfd, active_cache, active_nr) || - commit_index_file(&cache_file)) - die("Unable to write new cachefile"); + if (active_cache_changed) { + if (write_cache(newfd, active_cache, active_nr) || + commit_index_file(&cache_file)) + die("Unable to write new cachefile"); + } return has_errors ? 1 : 0; } ```