Re: Git status extremely slow if any file is a multiple of 8GBi

From: Philip Oakley <philipoakley@iee.email>
Date: May 5, 2022, 22:55 UTC
Message-ID: <9b6b7ad0-df90-62e0-abb1-c23862ded4ac@iee.email>
In-Reply-To: <0d78c98a-841e-719b-add3-acc7a7a2d7c6@web.de>

On 05/05/2022 22:04, René Scharfe wrote:

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> 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@pobox.com> 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.

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>> 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.

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>> 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