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From: Christian Couder <christian.couder@gmail.com>
To: Mike Hommey <mh@glandium.org>
Cc: Constantine <hi-angel@yandex.ru>, git <git@vger.kernel.org>
Subject: Re: git-clone causes out of memory
Date: Fri, 13 Oct 2017 12:44:52 +0200
Message-ID: <CAP8UFD3vniWZQ9Wb1oMo-bbj8n7CTjTHUNhBRwg6jN9x0+ApAQ@mail.gmail.com> (raw)
In-Reply-To: <20171013103722.rvr7536mu2hoo4wb@glandium.org>

On Fri, Oct 13, 2017 at 12:37 PM, Mike Hommey <mh@glandium.org> wrote:
> On Fri, Oct 13, 2017 at 12:26:46PM +0200, Christian Couder wrote:
>>
>> After cloning it with -n, there is the following "funny" situation:
>>
>> $ time git rev-list HEAD
>> 7af99c9e7d4768fa681f4fe4ff61259794cf719b
>> 18ed56cbc5012117e24a603e7c072cf65d36d469
>> 45546f17e5801791d4bc5968b91253a2f4b0db72
>>
>> real    0m0.004s
>> user    0m0.000s
>> sys     0m0.004s
>> $ time git rev-list HEAD -- d0/d0/d0/d0/d0/d0/d0/d0/d0/d0/f0
>>
>> real    0m0.004s
>> user    0m0.000s
>> sys     0m0.000s
>> $ time git rev-list HEAD -- d0/d0/d0/d0/d0/d0/d0/d0/d0/d0
>>
>> real    0m0.004s
>> user    0m0.000s
>> sys     0m0.000s
>> $ time git rev-list HEAD -- d0/d0/d0/d0/d0/d0/d0/d0/
>> 45546f17e5801791d4bc5968b91253a2f4b0db72
>>
>> real    0m0.005s
>> user    0m0.008s
>> sys     0m0.000s
>> $ time git rev-list HEAD -- d0/d0/d0/d0/d0/
>> 45546f17e5801791d4bc5968b91253a2f4b0db72
>>
>> real    0m0.203s
>> user    0m0.112s
>> sys     0m0.088s
>> $ time git rev-list HEAD -- d0/d0/d0/d0/
>> 45546f17e5801791d4bc5968b91253a2f4b0db72
>>
>> real    0m1.305s
>> user    0m0.720s
>> sys     0m0.580s
>> $ time git rev-list HEAD -- d0/d0/d0/
>> 45546f17e5801791d4bc5968b91253a2f4b0db72
>>
>> real    0m12.135s
>> user    0m6.700s
>> sys     0m5.412s
>>
>> So `git rev-list` becomes exponentially more expensive when you run it
>> on a shorter directory path, though it is fast if you run it without a
>> path.
>
> That's because there are 10^7 files under d0/d0/d0, 10^6 under
> d0/d0/d0/d0/, 10^5 under d0/d0/d0/d0/d0/ etc.
>
> So really, this is all about things being slower when there's a crazy
> number of files. Picture me surprised.
>
> What makes it kind of special is that the repository contains a lot of
> paths/files, but very few objects, because it's duplicating everything.
>
> All the 10^10 blobs have the same content, all the 10^9 trees that point
> to them have the same content, all the 10^8 trees that point to those
> trees have the same content, etc.
>
> If git wasn't effectively deduplicating identical content, the repository
> would be multiple gigabytes large.

Yeah, but perhaps Git could be smarter when rev-listing too and avoid
processing files or directories it has already seen?

  reply index

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-10-13  9:51 Constantine
2017-10-13 10:06 ` Mike Hommey
2017-10-13 10:26   ` Christian Couder
2017-10-13 10:37     ` Mike Hommey
2017-10-13 10:44       ` Christian Couder [this message]
2017-10-13 12:04         ` Junio C Hamano
2017-10-13 12:12           ` Constantine
2017-10-13 12:44             ` Jeff King
2017-10-13 13:15               ` Derrick Stolee
2017-10-13 13:39                 ` Derrick Stolee
2017-10-13 13:50                   ` Jeff King
2017-10-13 13:55                     ` Derrick Stolee
2017-10-13 13:56                       ` Jeff King
2017-10-13 14:10                         ` Jeff King
2017-10-13 14:20                           ` Jeff King
2017-10-13 14:25                             ` Derrick Stolee
2017-10-13 14:26                               ` Jeff King
2017-10-13 14:30                                 ` Derrick Stolee
2017-10-13 15:27                                 ` [PATCH] revision: quit pruning diff more quickly when possible Jeff King
2017-10-13 15:37                                   ` Derrick Stolee
2017-10-13 15:44                                     ` Jeff King
2017-10-14  2:43                                   ` Junio C Hamano
2017-10-13 12:35   ` git-clone causes out of memory Jeff King

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