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https://issues.apache.org/jira/browse/TS-1742?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13598489#comment-13598489
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Brian Geffon commented on TS-1742:
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hey weijin, you're saying that volatile is not required because we're doing the
CAS anyway? The only issue with that is when you reload couldn't you possibly
get stuck in an infinite loop of compare and swaps if the load was optimized
away because there wasn't the volatile keyword?
You guys definitely know the freelist stuff better than me, so if you think
it's safe to take out I'll definitely do it.
> 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, 128bit_cas.patch.2
>
>
> 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:
> {code}
> #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))
> {code}
> 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:
> {code}
> #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
> {code}
> 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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