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Re: Lockless Refs?

From
Junio C Hamano <gitster@pobox.com>
Date
Dec 28, 2012, 16:58 UTC
Message-ID
<7vlicijepv.fsf@alter.siamese.dyndns.org>
In-Reply-To
<201212271611.52203.mfick@codeaurora.org>
Martin Fick <mfick@codeaurora.org> writes:
Show 6 quoted lines
> 3) To create a ref, it must be renamed from the null file (sha 
> 0000...) to the new value just as if it were being updated 
> from any other value, but there is one extra condition: 
> before renaming the null file, a full directory scan must be 
> done to ensure that the null file is the only file in the 
> directory...

While you are scanning this directory to make sure it is empty, I am contemplating to create the same ref with a different value. You finished checking but haven't created the null. I have also scanned, created the null and renamed it to my value. Now you try to create the null, succeed, and then rename. We won't know which of the two non-null values are valid, but worse yet, I think one of them should have failed in the first place.

Sounds like we would need some form of locking around here. Is your goal "no locks", or "less locks"?

> I don't know how this new scheme could be made to work with 
> the current scheme,...

It is much more important to know if/why yours is better than the current scheme in the first place. Without an analysis on how the new scheme interacts with the packed refs and gives better behaviour, that is kinda difficult.

I think transition plans can wait until that is done. If it is not even marginally better, we do not have to worry about transitioning at all. If it is only marginally better, the transition has to be designed to be no impact to the existing repositories. If it is vastly better, we might be able to afford a flag day.

Previous: Martin FickNext: Martin Fick
Message 10 of 23 in “refs: do not use cached refs in repack_without_ref”
  1. refs: do not use cached refs in repack_without_refJeff King, Dec 21, 2012
  2. Michael HaggertyDec 26, 2012
  3. Lockless Refs? (Was [PATCH] refs: do not use cached refs in repack_without_ref)Martin Fick, Dec 27, 2012
  4. Martin FickDec 28, 2012
  5. Junio C HamanoDec 28, 2012
  6. Jeff KingDec 29, 2012
  7. Martin FickDec 29, 2012
  8. Jeff KingDec 29, 2012
  9. Martin FickDec 29, 2012
  10. Junio C HamanoDec 28, 2012
  11. Martin FickDec 29, 2012
  12. Jeff KingDec 29, 2012
  13. Martin FickDec 29, 2012
  14. Martin FickDec 30, 2012
  15. Martin FickDec 31, 2012
  16. Martin FickJan 3, 2013
  17. Pyeron, Jason J CTR (US)Jan 4, 2013
  18. Martin FickJan 4, 2013
  19. Junio C HamanoJan 4, 2013
  20. Jeff KingJan 5, 2013
  21. Drew NorthupJan 22, 2013
  22. Jeff KingDec 29, 2012
  23. Martin FickJan 7, 2013

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