This is the mail archive of the
mailing list for the libstdc++ project.
string and __thread
- From: Nathan Myers <ncm-nospam at cantrip dot org>
- To: libstdc++ at gcc dot gnu dot org
- Date: Fri, 28 Mar 2003 14:38:47 -0800
- Subject: string and __thread
As I understand it, much (most?) of the contention observed for
basic_string<> in heavily-threaded real-world programs occurs in fights
over access to the reference count on the (single) static empty-string
It occurs to me that we might declare that object "__thread", so there
would be as many as there are threads, reducing contention scalably as
the number of threads increases, and (as the number of threads exceeds
the number of CPUs) beyond.
This would not eliminate contention, as any thread might "use" the
empty-string object that is static to any other thread, but the
incidental traffic in increments and decrements to any one instance
would be much less.
I wonder if declaring it __thread makes it much more expensive to get to.
Another alternative would be to change the functions that increment
and decrement the reference count, and first compare the address they
propose scribbling against the known empty-string address, and then
just not change it when the address matches.
Experiments should be pretty easy to do. Is there anybody out there
who relies heavily on strings and lots of threads, who can perform some
ncm-nospam at cantrip dot org