Date: Fri, 27 Mar 2015 23:44:52 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: Rui Paulo <rpaulo@me.com> Cc: Eric van Gyzen <vangyzen@FreeBSD.org>, current@FreeBSD.org Subject: Re: SSE in libthr Message-ID: <20150327214452.GR2379@kib.kiev.ua> In-Reply-To: <3A96AAEC-9C1C-444E-9A73-3CD2AED33116@me.com> References: <5515AED9.8040408@FreeBSD.org> <3A96AAEC-9C1C-444E-9A73-3CD2AED33116@me.com>
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On Fri, Mar 27, 2015 at 01:49:03PM -0700, Rui Paulo wrote:
> On Mar 27, 2015, at 12:26, Eric van Gyzen <vangyzen@FreeBSD.org> wrote:
> >
> > In a nutshell:
> >
> > Clang emits SSE instructions on amd64 in the common path of
> > pthread_mutex_unlock. This reduces performance by a non-trivial amount. I'd
> > like to disable SSE in libthr.
> >
> > In more detail:
> >
> > In libthr/thread/thr_mutex.c, we find the following:
> >
> > #define MUTEX_INIT_LINK(m) do { \
> > (m)->m_qe.tqe_prev = NULL; \
> > (m)->m_qe.tqe_next = NULL; \
> > } while (0)
> >
> > In 9.1, clang 3.1 emits two ordinary mov instructions:
> >
> > movq $0x0,0x8(%rax)
> > movq $0x0,(%rax)
> >
> > Since 10.0 and clang 3.3, clang emits these SSE instructions:
> >
> > xorps %xmm0,%xmm0
> > movups %xmm0,(%rax)
> >
> > Although these look harmless enough, using the FPU can reduce performance by
> > incurring extra overhead due to context-switching the FPU state.
> >
> > As I mentioned, this code is used in the common path of pthread_mutex_unlock. I
> > have a simple test program that creates four threads, all contending for a
> > single mutex, and measures the total number of lock acquisitions over several
> > seconds. When libthr is built with SSE, as is current, I get around 53 million
> > locks in 5 seconds. Without SSE, I get around 60 million (13% more). DTrace
> > shows around 790,000 calls to fpudna versus 10 calls. There could be other
> > factors involved, but I presume that the FPU context switches account for most
> > of the change in performance.
> >
> > Even when I add some SSE usage in the application--incidentally, these same
> > instructions--building libthr without SSE improves performance from 53.5 million
> > to 55.8 million (4.3%).
> >
> > In the real-world application where I first noticed this, performance improves
> > by 3-5%.
> >
> > I would appreciate your thoughts and feedback. The proposed patch is below.
> >
> > Eric
> >
> >
> >
> > Index: base/head/lib/libthr/arch/amd64/Makefile.inc
> > ===================================================================
> > --- base/head/lib/libthr/arch/amd64/Makefile.inc (revision 280703)
> > +++ base/head/lib/libthr/arch/amd64/Makefile.inc (working copy)
> > @@ -1,3 +1,8 @@
> > #$FreeBSD$
> >
> > SRCS+= _umtx_op_err.S
> > +
> > +# Using SSE incurs extra overhead per context switch,
> > +# which measurably impacts performance when the application
> > +# does not otherwise use FP/SSE.
> > +CFLAGS+=-mno-sse
>
> Good catch!
>
> Regarding your patch, I think we should disable even more, if possible. How about:
>
> CFLAGS+= -mno-mmx -mno-3dnow -mno-sse -mno-sse2 -mno-sse3
I think so.
Also, this should be done for libc as well, both on i386 and amd64.
I am not sure, should compiler-rt be included into the set ?
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