On Fri, Jul 06, 2018 at 02:22:14AM +0000, Liu, Yong wrote: > > > > -----Original Message----- > > From: dev [mailto:dev-boun...@dpdk.org] On Behalf Of Sachin Saxena > > Sent: Thursday, July 05, 2018 10:46 PM > > To: Liu, Yong <yong....@intel.com>; Yang, Zhiyong <zhiyong.y...@intel.com>; > > tho...@monjalon.net; dev@dpdk.org > > Subject: Re: [dpdk-dev] [PATCH] mk: using initial-exec model for thread > > local variable > > > > > > > > > > > > When building share library, thread-local storage model will be changed > > to > > > global-dynamic. It will add additional cost for reading thread local > > variable. > > > On the other hand, dynamically load share library with static TLS will > > request > > > additional DTV slot which is limited by loader. By now only > > librte_pmd_eal.so > > > contain thread local variable. So that can make TLS model back to > > initial-exec > > > like static library for better performance. > > > > > > Signed-off-by: Marvin Liu <yong....@intel.com> > > > > > > diff --git a/mk/toolchain/gcc/rte.vars.mk b/mk/toolchain/gcc/rte.vars.mk > > > index 7e4531bab..19d5e11ef 100644 > > > --- a/mk/toolchain/gcc/rte.vars.mk > > > +++ b/mk/toolchain/gcc/rte.vars.mk > > > @@ -43,6 +43,13 @@ ifeq (,$(findstring -O0,$(EXTRA_CFLAGS))) endif > > endif > > > > > > +# Initial execution TLS model has better performane compared to dynamic > > > +# global. But this model require for addtional slot on DTV when dlopen > > > +# object with thread local variable. > > > +ifeq ($(CONFIG_RTE_BUILD_SHARED_LIB),y) > > > +TOOLCHAIN_CFLAGS += -ftls-model=initial-exec endif > > > + > > > > [Sachin Saxena] Using initial-exec model for shared object is not > > recommended. If you link a shared object containing IE-model, the object > > will have the DF_STATIC_TLS flag set. By the spec, this means that dlopen() > > might refuse to load it if TLS usage is greater than static TLS space. > > This is what happening, when I tried to validate this change on ARM64 > > based NXP platform with VPP-dpdk solution. VPP initialization fails with > > following error: > > "load_one_plugin:145: /usr/lib/vpp_plugins/dpdk_plugin.so: cannot > > allocate memory in static TLS block" > > > > Note that dpdk dpaa2 driver and VPP both uses TLS variables quite > > significantly. When forced to Initial-exec model in dpdk shared object, > > VPP static TLS space is getting exhausted and dlopen() returns error while > > trying to load the DPDK object. > > For same reason, when we use "-fPIC" the default TLS model changed to > > "global-dynamics" from "Initial-exec". > > > > In my opinion, this change should not be merged as it is breaking basic > > functionality. > > Thanks for your opinion, Sachin. > IE model may cause problem when using dlopen open share object. On the other > hand, it can benefit performance. > It will be better to keep current workable setting and users may change it by > themselves. > What is the performance delta, and where is it most seen? I suggest for future patches like this, that the commit message itself should give a rough/approx indication of the perf impacts.
/Bruce
