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https://issues.apache.org/jira/browse/TS-1742?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13598501#comment-13598501
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John Plevyak commented on TS-1742:
----------------------------------
We can take it out because we do the loads manually for the cas.
I always liked to use volatile as a marker that the variable was being
accessed outside
of a lock, but if it is causing a performance problem then we could convert
the keyword
into a comment:
// Warning: this variable is read and written in multiple threads without a
lock, use INK_QUEUE_LD to read safely.
john
> 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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