Subject: Re: Windows performance / threading file access

From: Karsten Blees <[email-masked]>
Date: Fri, 11 Oct 2013 02:51:28 +0200

Am 10.10.2013 22:19, schrieb Sebastian Schuberth:
> Please keep in mind to CC the msysgit mailing list for Windows-specific stuff. I'm also CC'ing Karsten who has worked on performance improvements for Git for Windows in the past.
> 

Thanks

> Thanks for bringing this up!
> 
> -- 
> Sebastian Schuberth
> 
> 
>> Hi folks,
>>
>> I don't follow the mailing list carefully, so forgive me if this has
>> been discussed before, but:
>>
>> I've noticed that when working with a very large repository using msys
>> git, the initial checkout of a cloned repository is excruciatingly
>> slow (80%+ of total clone time).  The root cause, I think, is that git
>> does all the file access serially, and that's really slow on Windows.
>>

What exactly do you mean by "excruciatingly slow"?

I just ran a few tests with a big repo (WebKit, ~2GB, ~200k files). A full checkout with git 1.8.4 on my SSD took 52s on Linux and 81s on Windows. Xcopy /s took ~4 minutes (so xcopy is much slower than git). On a 'real' HD (WD Caviar Green) the Windows checkout took ~9 minutes.

That's not so bad I think, considering that we read from pack files and write both files and directory structures, so there's a lot of disk seeking involved.

If your numbers are much slower, check for overeager virus scanners and probably the infamous "User Account Control" (On Vista/7 (8?), the luafv.sys driver slows down things on the system drive even with UAC turned off in control panel. The driver can be disabled with "sc config luafv start= disabled" + reboot. Reenable with "sc config luafv start= auto").

>> Has anyone considered threading file access to speed this up?  In
>> particular, I've got my eye on this loop in unpack-trees.c:
>>

Its probably worth a try, however, in my experience, doing disk IO in parallel tends to slow things down due to more disk seeks.

I'd rather try to minimize seeks, e.g.:

* read the blob data for a block of cache_entries, then write out the files, repeat (this would require lots of memory, though)

* index->cache is typically sorted by name and pack files by size, right? Perhaps its faster to iterate cache_entries by size so that we read the pack file sequentially (but then we'd write files/directories in random order...)

If you want to measure exactly which part of checkout eats the performance, check out this: https://github.com/kblees/git/commits/kb/performance-tracing-v3

Bye,
Karsten

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