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Re: Consensus: Tuning runtime behaviour with environment variables.

On May 29, 2013, OndÅej BÃlka <> wrote:

> When performance is concerned I am for auto tuning without env variables
> unless performance depends on external factors.

Here's one case where I've long entertained adding an env var to
configure libc's internal behavior: the thread local storage
optimizations I implemented years ago attempt to assign even the thread
local area of dlopened libraries to the static tls segment, so that they
can be accessed more efficiently.

The problem is that, unlike the transitive closure of DT_NEEDED dynamic
libraries for a program, that can all be loaded before computing the
total amount of tls they require, dlopened libraries are loaded

What I wanted to do was to introduce a tunable for glibc to reserve an
additional amount of space to the static TLS segment, so that TLS
segments in the dlopened modules could be assigned to the static
segment, and relocations to their thread-local variables could be
resolved to the most efficient implementation.

How else could we enable *users* to tune the amount of thread-local
storage to reserve in the static TLS segment for dlopened modules?

Note that I write users, not developers, because the developers of the
program may have no idea whatsoever about what plugins individual users
would use.  Requiring developers to modify the program (e.g., so as to
add some extra note to the executable) is not a reasonable solution.
Also, since the static segment size has to be determined before control
is relinquished by the dynamic loader or the static executable start-up
code to the program, this is not the sort of tunable that could possibly
be set up by the program itself, say after reading a configuration file.

Alexandre Oliva, freedom fighter
You must be the change you wish to see in the world. -- Gandhi
Be Free! --   FSF Latin America board member
Free Software Evangelist      Red Hat Brazil Compiler Engineer

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