flush xorg image cache?

Roland Mainz roland.mainz at nrubsig.org
Sat Apr 23 16:33:38 PDT 2005

Alan Cox wrote:
> On Mer, 2005-04-20 at 17:59, Roland Mainz wrote:
> > heap size - and due the implementation of most default memory allocators
> > in unix/linux the heap size cannot shrink[1].
> ITYM "In unix"
> To quote the glibc manual on "Efficiency Considerations for 'malloc'"
> --
>    Very large blocks (much larger than a page) are allocated with
> `mmap' (anonymous or via `/dev/zero') by this implementation.  This has
> the great advantage that these chunks are returned to the system
> immediately when they are freed.  Therefore, it cannot happen that a
> large chunk becomes "locked" in between smaller ones and even after
> calling `free' wastes memory.  The size threshold for `mmap' to be used
> can be adjusted with `mallopt'.  The use of `mmap' can also be disabled
> completely.
> --
> So for the Linux case mallopt should be all you need to touch.

That's for Linux/glibc but not for all the other OSes. Additionally only
relatively long-living memory such as pixmaps should be allocated using
|mmap()| as this always results in a syscall (AFAIK Jim Gettys said some
while ago that the Xserver code is a good exercise in avoiding
syscalls... :) and may require some additional adjustments (for example
I'd like to force the usage of large pages (64K, 512K, 4M) for pixmap
memory to save some entries in the TLB).



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