rfc:tls

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rfc:tls [2008/08/26 13:55] – tested on recent windows lbarnaudrfc:tls [2008/08/26 19:04] – . lbarnaud
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 Unfortunately the TLS model used in these tests was the static model, which is restrictive, and in particular does not allow to use it in shared libraries which will be loaded dynamically. (c.f [[tls#tls_internals|TLS internals]] bellow). Unfortunately the TLS model used in these tests was the static model, which is restrictive, and in particular does not allow to use it in shared libraries which will be loaded dynamically. (c.f [[tls#tls_internals|TLS internals]] bellow).
  
-For the PHP module to be able to be loaded at runtime in Apache or an other server with this patch enabled, it has to be built with the static TLS model (on gcc, -ftls-model=global-dynamic), which also requires to build position independent code. While the last makes a big difference on IA-32, this is the default on x86_64 and the results on bench.php on an unpatched PHP are the same as IA-32/PIC builds.+For the PHP module to be able to be loaded at runtime in Apache or an other server with this patch enabled, it has to be built with the dynamic TLS model (on gcc, -ftls-model=global-dynamic), which also requires to build position independent code. While the last makes a big difference on IA-32, this is the default on x86_64 and the results on bench.php on an unpatched PHP are the same as IA-32/PIC builds.
  
 Native TLS can be enabled with %%--with-tsrm-__thread-tls%% or %%--with-tsrm-full__thread-tls%%. The last declares globals statically instead of making them pointers. Native TLS can be enabled with %%--with-tsrm-__thread-tls%% or %%--with-tsrm-full__thread-tls%%. The last declares globals statically instead of making them pointers.
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 ===== Windows ===== ===== Windows =====
  
-Dynamically loaded DLLs can use TLS starting with Windows Vista and Server 2008. But there is a restriction: TLS variables can't be exported, which means that they can't be accessed outside of the DLL. CLI and ISAPI SAPIs works with TLS enabled, but they must be built like this is done on other platforms, with all code embeded in the executable/library. The same apply for extension, they must be built statically in PHP.+Dynamically loaded DLLs can use TLS starting with Windows Vista and Server 2008. But there is a restriction: TLS variables can't be exported, which means that they can't be accessed outside of the DLL. CLI and ISAPI SAPIs works with TLS enabled, but they must be built like this is done on other platforms, with all code embeded in the executable/library (instead of a separate php5ts.dll linked by SAPIs). The same apply for extension, they must be built statically in PHP.
  
 ===== TLS internals ===== ===== TLS internals =====
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 ==== Static model ==== ==== Static model ====
  
-Each block is allocated at a fixed (linker-defined) offset from an address specific to each thread. As this address can be accessed very quickly, this allows very quick access to each TLS block. For instance, on Linux/IA-32, this thread-specific-address is the Thread Control Block, whose address is stored in offset 0 of the %gs segment register.+Each block is allocated at a fixed (loader-defined) offset from an address specific to each thread. As this address can be accessed very quickly, this allows very quick access to each TLS block. For instance, on Linux/IA-32, this thread-specific-address is the Thread Control Block, whose address is stored in offset 0 of the %gs segment register.
  
 The way the static model works requires that the memory needed by each TLS variable to be allocated before program startup. This means that the static model can not be used in shared libraries loaded at runtime.  The way the static model works requires that the memory needed by each TLS variable to be allocated before program startup. This means that the static model can not be used in shared libraries loaded at runtime. 
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 Linux, Solaris, FreeBSD, Windows. Linux, Solaris, FreeBSD, Windows.
  
-Linux, Solaris and FreeBSD implementations allocate a fixed amount of surplus memory especially to allow dynamically loaded libraries to use the static model. Linux allocates 1664 bytes, FreeBSD 64 and Solaris 512. This amount of memory is always allocated in addition of the memory allocated for TLS before program startup, and is always available (this memory can be used only by dlopen()ed modules using static TLS).+Linux, Solaris and FreeBSD implementations allocate a fixed amount of surplus memory especially to allow dynamically loaded libraries to use the static model. Linux allocates 1664 bytes, FreeBSD 64 and Solaris 512. This amount of memory is always allocated in addition of the memory allocated for TLS before program startup, and is always available (this memory can be used only by dlopen()ed modules using static TLS). These behaviors are undocumented (except by comments in Linux and FreeBSD loaders/linkers code). This has been tested with a test program and verified by reading the relevant code on Linux and FreeBSD.
  
 On GCC this model can be selected by using -ftls-model=initial-exec. On SunStudio: -xthreadvar=no%dynamic. For both, this model is the default one when building non-PIC code. On GCC this model can be selected by using -ftls-model=initial-exec. On SunStudio: -xthreadvar=no%dynamic. For both, this model is the default one when building non-PIC code.
rfc/tls.txt · Last modified: 2017/09/22 13:28 by 127.0.0.1