On 05/05/2022 22:04, René Scharfe wrote: > Am 04.05.22 um 19:47 schrieb Jason Hatton: >>>> The condition sd_size==0 is used as a signal for "no, we really need >>>> to compare the contents", and causes the contents to be hashed, and >>>> if the contents match the object name recorded in the index, the >>>> on-disk size is stored in sd_size and the entry is marked as >>>> CE_UPTODATE. Alas, if the truncated st_size is 0, the resulting >>>> entry would have sd_size==0 again, so a workaround like what you >>>> outlined is needed. >>> Junio C Hamano <gitster@xxxxxxxxx> writes: >>> >>> This is of secondary importance, but the fact that Jason observed >>> 8GBi files gets hashed over and over unnecessarily means that we >>> would do the same for an empty file, opening, reading 0-bytes, >>> hashing, and closing, without taking advantage of the fact that >>> CE_UPTODATE bit says the file contents should be up-to-date with >>> respect to the cached object name, doesn't it? >>> >>> Or do we have "if st_size == 0 and sd_size == 0 then we know what it >>> hashes to (i.e. EMPTY_BLOB_SHA*) and there is no need to do the >>> usual open-read-hash-close dance" logic (I didn't check)? >> Junio C Hamano >> >> As best as I can tell, it rechecks the zero sized files. My Linux box can run >> git ls in .006 seconds with 1000 zero sized files in the repo. Rehashing every >> file that is a multiple of 2^32 with every "git ls" on the other hand... >> >> I managed to actually compile git with the proposed changes. > Meaning that file sizes of n * 2^32 bytes get recorded as 1 byte instead > of 0 bytes? Why 1 and not e.g. 2^32-1 or 2^31 (or 42)? My thought on this. after considering a few options, would be that the 'sign bit' of the uint32_t size should be set to 1 when the high word of the 64 bit filesize value is non zero. This would result in file sizes of 0 to 4GiB-1 retaining their existing values, and those from 4GiB onward produces a down-folded 2GiB to 4GiB-1 values. This would mean, That we are able to detect almost all incremental and decremental changes in filesizes, as well as retaining the 'zero is racy' flag aspect. >> It seems to correct >> the problem and "make test" passes. If upgrading to the patched version if git, >> git will rehash the 8GBi files once and work normally. If downgrading to an >> unpatched version, git will perceive that the 8GBi files have changes. This >> needs to be corrected with "git add" or "git checkout". > Not nice, but safe. Can there be an unsafe scenario as well? Like if a > 4GiB file gets added to the index by the new version, which records a > size of 1, then the file is extended by one byte while mtime stays the > same and then an old git won't detect the change? There is still some potential for different Git versions to be 'confused' for these very large files, but I feel that it's relatively safe (no worse than the 'set to unity' idea). For large files we will always have that loss of precision at the 32bit rollover. It just a case of choosing a least worst. I haven't considered if my proposed 'truncation' overhead would be fast code. >> If you people are >> interested, I may be able to find a way to send a patch to the list or put it >> on github. > Patches are always welcome, they make discussions and testing easier. > > René Philip
