Date: Mon, 29 Mar 2004 17:36:08 -0500 (EST) From: Daniel Eischen <eischen@vigrid.com> To: Doug Rabson <dfr@nlsystems.com> Cc: freebsd-threads@freebsd.org Subject: Re: Thread Local Storage Message-ID: <Pine.GSO.4.10.10403291718320.29683-100000@pcnet5.pcnet.com> In-Reply-To: <200403292316.17288.dfr@nlsystems.com>
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On Mon, 29 Mar 2004, Doug Rabson wrote: > On Monday 29 March 2004 22:56, Daniel Eischen wrote: > > On Mon, 29 Mar 2004, Doug Rabson wrote: > > > On Monday 29 March 2004 22:26, Daniel Eischen wrote: > > > > On Mon, 29 Mar 2004, Doug Rabson wrote: > > > > > Surely the GNU TLS ABI is preferable? It generates much smaller > > > > > code and needs many fewer relocations. > > > > > > > > No, we don't want an LDT for every thread and don't want > > > > to force a syscall for a thread switch. > > > > > > But the code it generates is at least twice the size for dynamic > > > TLS. It seems that the GNU people have done a better job defining > > > the TLS abi for i386. > > > > About the only thing that uses TLS that I know is nvidia's > > openGL. If you design an API correctly, there's no need > > for TLS. I would hope that it's usage would be limited. > > I'd quite like to see us use it for stuff like errno, _res and other > uglification currently in libc. Not until the 6.x timeframe though. I'd like to see libc free of TLS ;-) The _res stuff can be avoided by modifying the implementation to use thread-safe APIs. The current _res stuff can _almost_ be eliminated by passing using pthread_getspecific() once and passing the _res around internal APIs. That's actually a pretty simple change. > > > > > > You don't need a syscall at thread switch if you do something like: > > > > > > _thread_switch(...) > > > { > > > if (tcb doesn't have LDT entry) { > > > if (!free LDT entries) > > > steal LDT entry from non-running thread; > > > allocate LDT entry and point it at TLS goop for tcb. > > > } > > > load_gs(tcb's LDT sel); > > > > That's a system call on amd64. > > I'm not quite up to speed on amd64. So in 64-bit mode it doesn't really > have an LDT at all, is that right? I'm not sure, but you have to make a system call to set it or it's equivalent (amd64_set_fsbase()). -- Dan Eischen
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