On Thu, May 16, 2019 at 02:30:56PM -0700, Jonathan Tan wrote: > > So I could go either way, though I do think it makes sense for on-demand > > fetches for partial clones to avoid asking for thin packs as a general > > principle. > > This should not be a problem since fetch-pack can already know that > we're doing an on-demand fetch (args->no_dependents), so we should be > able to either plumb a "no-thin-pack" arg in the same way or rename > args->no_dependents to also encompass the no-thin-pack option. But this > can be done separately from this patch set, I think. Yeah, I think it can be done separately. Though the two may intermingle if we want to instruct index-pack that it should not try to pre-fetch if we did not ask for a thin pack. > > As a matter of fact, should partial clones _always_ avoid > > asking for thin packs? That would make this issue go away entirely. > > > > Sometimes it would be more efficient (we do not have to get an extra > > base object just to resolve the delta we needed) but sometimes worse (if > > we did actually have the base, it's a win). Whether it's a win would > > depend on the "hit" rate, and I suspect that is heavily dependent on > > workload characteristics (what kind of filtering is in use, are we > > topping up in a non-partial way, etc). > > I think it's best if we still allow servers to serve thin packs. For > example, if we're excluding only large blobs, clients would still want > servers to be able to delta against blobs that they have. Yes, this is getting into the hit-rate thing I mentioned. You're right that for a reasonably typical case of "no blobs over 10MB" we'd have a very high hit rate, and disabling thin packs would almost certainly be a big loss. I guess even when we have a "miss", the cost is usually not that high either. If we get A as a delta against B, then in the non-thin-pack case we transfer all of A. In the thin-pack case with pre-fetch we transfer all of B, and then the delta. But the delta is often small enough compared to the total content that it's not that big a deal either way. There are pathological cases, of course, but that's already true. :) So you're right, it's probably still a win to use thin packs when we can. > > Right, REF_DELTA is definitely correctly handled currently, and I don't > > think that would break with your patch. It's just that your patch would > > introduce a bunch of extra traffic as we request bases separately that > > are already in the pack. > > Ah...I see. For this problem, I think that it can be solved with the > "if (objects[d->obj_no].real_type != OBJ_REF_DELTA)" check that the > existing code uses before calling read_object(). I'll include this in > the next reroll if any other issue comes up. I'm confused about this. Aren't we pre-fetching before we've actually resolved deltas? The base could be in the pack as a true base, and we might have seen it already then. But it could itself be a delta, and we wouldn't know we have it until we resolve it (this gets into the lucky/unlucky ordering thing). -Peff
