On Thu, Jan 28, 2021 at 07:53:34PM -0500, Jeff King wrote: > On Mon, Jan 25, 2021 at 06:37:46PM -0500, Taylor Blau wrote: > > > When an on-disk reverse index exists, there is no need to generate one > > in memory. In fact, doing so can be slow, and require large amounts of > > the heap. > > > > Let's make sure that we treat the on-disk reverse index with precedence > > (i.e., that when it exists, we don't bother trying to generate an > > equivalent one in memory) by teaching Git how to conditionally die() > > when generating a reverse index in memory. > > > > Then, add a test to ensure that when (a) an on-disk reverse index > > exists, and (b) when setting GIT_TEST_REV_INDEX_DIE_IN_MEMORY, that we > > do not die, implying that we read from the on-disk one. > > I don't love this kind of hackery, as it will have to live forever if we > want to keep testing this feature. But I also don't know how else to > tell in the regular test suite that we are avoiding the slow path. > > Would it be enough to instead add a t/perf test which checks the speed > of: > > echo HEAD | git cat-file --batch-check='%(objectsize:disk)' > > ? I hate relying on the perf suite, because it gets run way less often > (and requires a lot of squinting to interpret the results). But it > wouldn't require any extra code the binary, as it's observing the actual > user-visible thing we care about: speed. > > (I guess we care as much or more about peak heap usage, but measuring > that is a whole other can of worms). Yeah, I think that the thing we care most about is peak RSS, but I agree that I don't really want to wade into figuring out how to measure that reliably during test time (I think there is a separate and less relevant argument about whether that is something that we should be testing or not). But, getting back to your original comment, I think that I actually prefer to have the GIT_TEST_XYZ_DIE stuff in the binary than I do relying on the perf suite to catch stuff like this. I understand your concern about the binary size. I guess you could #ifdef this out and only build it in during the test suite, but then you're testing a different binary, and so that calls into question the integrity of the test suite itself, etc. etc. So, I guess that's all to say that I while I do find this to be hack-y and gross, I don't think that it's all that bad when you compare it to the alternatives. As usual, I'm happy to change it if you feel strongly that it should be changed. > -Peff Thanks, Taylor
