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Re: Failure to dlopen libgomp due to static TLS data


On Thu, Feb 12, 2015 at 06:23:12PM +0000, Andrew Haley wrote:
> On 02/12/2015 04:16 PM, Rich Felker wrote:
> > On Thu, Feb 12, 2015 at 05:11:45PM +0100, Jakub Jelinek wrote:
> >> On Thu, Feb 12, 2015 at 11:09:59AM -0500, Rich Felker wrote:
> >>>
> >>> This usage is supposed to be deprecated. Why isn't libgomp using
> >>> TLSDESC/gnu2 model?
> >>
> >> Because it is significantly slower.
> > 
> > Seems very unlikely. If storage is allocated in static TLS, TLSDESC is
> > almost indistinguishable from IE in performance, even when you run
> > artificial benchmarks that do nothing but hammer TLS access. When it
> > gets allocated in dynamic TLS, it's somewhat slower, but still
> > unlikely to matter for most usage IMO.
> 
> The problem I'm seeing is that dynamic TLS is always used even when not
> necessary, and that hurts Java (which accesses TLS 128k times in the first
> 500ms or so of execution).  According to lxo his patch fixes that.

Given those numbers, each access would need to be taking 38ns to
consume even 1% of the cpu time being spent. I would guess accesses
are closer to 5ns for TLSDESC in static area and 10-15ns for dynamic.
So I don't think this is a botteneck.

Rich


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