Alan Coopersmith wrote:
Joerg Schilling wrote:
Another issue: Xsun did use mmap() to allocate bigger parts of memory in the
X server. If big memory leaking program like netscape died in former times,
Xsun did shrink. Today, we have firefox that itself allocates less memory than
before but rather forxes X to allocate more memory and Xorg is malloc() based
and this does not shrink when fireforx dies.......
We had to turn down Xsun's use of mmap() because it caused performance problems
when programs like Netscape or the GNOME desktop had allocated hundreds of
pixmaps per session, and you had dozens of sessions running on a Sun Ray server
leading to TLB thrashing for all those new page mapping entries, especially on
the UltraSPARC CPU's with small TLBs. There were also performance issues with
pixmap allocation & deallocation becoming much more expensive, since
mmap/mmunmap are far more expensive calls than malloc/free. (See for instance,
the GNOME stock ticker issue described in the original DTrace Usenix paper,
and http://bugs.opensolaris.org/bugdatabase/view_bug.do?bug_id=4757131 )
Even so, we've looked at porting those changes to Xorg before, and I released
the Xsun code to Roland at one point when he volunteered to port it, but have
never gotten it done.
What is needed is allocation technology that doesn't muck w/ the process
address space because that's expensive for long-running processes that
have migrated to every cpu on the box; you end up cross-calling lots of
other cpus, and if your clients do stupid allocation behaviors the
cross call rate goes through the roof. On the other hand, if the pages
are no longer interesting, writing them out to swap and paging them back
in isn't very interesting either.
What may be a useful experiment is using
madvise(caddr_t addr, size_t len, MADV_FREE)
on large deallocated pixmaps:
MADV_FREE Tell the kernel that contents in the
specified address range are no longer
important and the range will be
overwritten. When there is demand for
memory, the system will free pages
associated with the specified address
range. In this instance, the next
time a page in the address range is
referenced, it will contain all
zeroes. Otherwise, it will contain
the data that was there prior to the
MADV_FREE call. References made to
the address range will not make the
system read from backing store (swap
space) until the page is modified
again.
- Bart
--
Bart Smaalders Solaris Kernel Performance
[email protected] http://blogs.sun.com/barts
"You will contribute more with mercurial than with thunderbird."
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