On Fri, Jul 20, 2018 at 7:40 PM Jeff King <peff@xxxxxxxx> wrote: > > On Fri, Jul 20, 2018 at 05:39:43PM +0200, Nguyễn Thái Ngọc Duy wrote: > > > Let's start with some background about oe_delta_size() and > > oe_set_delta_size(). If you already know, skip the next paragraph. > > > > These two are added in 0aca34e826 (pack-objects: shrink delta_size > > field in struct object_entry - 2018-04-14) to help reduce 'struct > > object_entry' size. The delta size field in this struct is reduced to > > only contain max 2MB. So if any new delta is produced and larger than > > 2MB, it's dropped because we can't really save such a large size > > anywhere. Fallback is provided in case existingpackfiles already have > > large deltas, then we can retrieve it from the pack. > > Minor nit, but isn't this 1MB (it was 2MB after one of your patches, but > I think v2.18.0 has 20 bits)? Argh.. I think I thought "2 ** 20" in my mind then typed "2 << 20" in python. And I thought I made a mistake in my previous commit message because it does mention 1MB... > > With this, we do not have to drop deltas in try_delta() anymore. Of > > course the downside is we use slightly more memory, even compared to > > 2.17.0. But since this is considered an uncommon case, a bit more > > memory consumption should not be a problem. > > I wondered how common this might be. The easiest way to see the largest > delta sizes is: > > git cat-file --batch-all-objects \ > --batch-check='%(objectsize:disk) %(deltabase)' | > grep -v 0000000000000000000000000000000000000000 | > sort -rn | head > > The biggest one in the kernel is ~300k. Which is about what I'd expect > for a normal source code repo. Even some private repos I have with a lot > of binary artifacts top out at about 3MB. So the new 32MB is probably I'll keep these numbers in v2 commit message, easier to find later. > > [1] With a small tweak. 2.17.0 on 64-bit linux can hold 2^64 byte > > deltas, which is absolutely insane. But windows builds, even > > 64-bit version, only hold 2^32. So reducing it to 2^32 should be > > quite safe. > > I'm not sure I completely agree with this. While 4GB deltas should be > pretty rare, the nice thing about 64-bit is that you never have to even > think about whether the variable is large enough. I think the 2^32 > limitations on Windows are something we should be fixing in the long > term (though there it is even worse because it is not just deltas, but > entire objects). I guess that means we stick to uint64_t then? It does increase more memory on 32-bit architecture (and probably processing cost as well because 64-bit uses up 2 registers). > > @@ -2278,6 +2274,8 @@ static void init_threaded_search(void) > > pthread_mutex_init(&cache_mutex, NULL); > > pthread_mutex_init(&progress_mutex, NULL); > > pthread_cond_init(&progress_cond, NULL); > > + pthread_mutex_init(&to_pack.lock, NULL); > > + to_pack.lock_initialized = 1; > > old_try_to_free_routine = set_try_to_free_routine(try_to_free_from_threads); > > } > > This is new in this iteration. I guess this is to cover the case where > we are built with pthread support, but --threads=1? If you mean the "lock_initialized" variable, not really. the other _lock() macros in builtin/ call pthread_mutex_lock() unconditionally, which is fine. But I feel a bit uncomfortable doing the same in pack-objects.h which technically is library code (but yes practically is a long arm of builtin/pack-objects.c), so lock_initialized is there to make sure we don't touch uninitialized locks if somebody forgets to init them first. > Given that we no longer have to touch this lock during the realloc, is > it worth actually putting it into to_pack? Instead, we could keep it > local to pack-objects, alongside all the other locks (and use the > lock_mutex() helper which handles the single-thread case). You probably notice the lock name is not "delta_size_lock". I intended to reuse this for locking other fields in struct packing_data as well. But that might be a bad idea. I have no strong opinion about this, so if we still end up locking the whole functions, I'll just move the lock back close to the others in builtin/pack-objects.c > Your original patch had to copy the oe_* helpers into the file to handle > that. But I think we're essentially just locking the whole functions: I'll try to avoid this lock when deltas are small and see if it helps the linux.git case on Elijah's machine. So we may end up locking just a part of these functions. -- Duy
