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diff --git a/manual/tunables.texi b/manual/tunables.texi
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@@ -250,6 +250,23 @@ increase the per-thread memory usage as necessary, so this tunable does
not need to be changed to allow many audit modules e.g. via @env{LD_AUDIT}.
@end deftp
+@deftp Tunable glibc.rtld.optional_static_tls
+Sets the amount of surplus static TLS in bytes to allocate at program
+startup. Every thread created allocates this amount of specified surplus
+static TLS. This is a minimum value and additional space may be allocated
+for internal purposes including alignment. Optional static TLS is used for
+optimizing dynamic TLS access for platforms that support such optimizations
+e.g. TLS descriptors or optimized TLS access for POWER (@code{DT_PPC64_OPT}
+and @code{DT_PPC_OPT}). In order to make the best use of such optimizations
+the value should be as many bytes as would be required to hold all TLS
+variables in all dynamic loaded shared libraries. The value cannot be known
+by the dynamic loader because it doesn't know the expected set of shared
+libraries which will be loaded. The existing static TLS space cannot be
+changed once allocated at process startup. The default allocation of
+optional static TLS is 512 bytes and is allocated in every thread.
+@end deftp
+
+
@node Elision Tunables
@section Elision Tunables
@cindex elision tunables