[jira] [Assigned] (TS-1742) Freelists to use 64bit version w/ Double Word Compare and Swap

["Brian Geffon (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/TS-1742?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Brian Geffon reassigned TS-1742:
--------------------------------

    Assignee: Brian Geffon
    
> Freelists to use 64bit version w/ Double Word Compare and Swap
> --------------------------------------------------------------
>
>                 Key: TS-1742
>                 URL: https://issues.apache.org/jira/browse/TS-1742
>             Project: Traffic Server
>          Issue Type: Improvement
>            Reporter: Brian Geffon
>            Assignee: Brian Geffon
>         Attachments: 128bit_cas.patch
>
>
> So to those of you familiar with the freelists you know that it works this way the head pointer uses the upper 16 bits for a version to prevent the ABA problem. The big drawback to this is that it requires the following macros to get at the pointer or the version:
> #define FREELIST_POINTER(_x) ((void*)(((((intptr_t)(_x).data)<<16)>>16) | \
>  (((~((((intptr_t)(_x).data)<<16>>63)-1))>>48)<<48)))  // sign extend
> #define FREELIST_VERSION(_x) (((intptr_t)(_x).data)>>48)
> #define SET_FREELIST_POINTER_VERSION(_x,_p,_v) \
>   (_x).data = ((((intptr_t)(_p))&0x0000FFFFFFFFFFFFULL) | (((_v)&0xFFFFULL) << 48))
> Additionally, since this only leaves 16 bits it limits the number of versions you can have, well more and more x86_64 processors support DCAS (double word compare and swap / 128bit CAS). This means that we can use 64bits for a version which basically makes the versions unlimited but more importantly it takes those macros above and simplifies them to:
> #define FREELIST_POINTER(_x) (_x).s.pointer
> #define FREELIST_VERSION(_x) (_x).s.version
> #define SET_FREELIST_POINTER_VERSION(_x,_p,_v) \
> (_x).s.pointer = _p; (_x).s.version = _v
> As you can imagine this will have a performance improvement, in my simple tests I measured a performance improvement of around 6%. Unfortunately, I'm not an expert with this stuff and I would really appreciate more community feedback before I commit this patch.
> Note: this only applies if you're not using a reclaimable freelist.

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