More (?) steps toward jemalloc within Cygwin DLL
Fri Jul 3 10:11:15 GMT 2020
On Jul 2 23:57, Mark Geisert wrote:
> Hi Corinna,
> Corinna Vinschen wrote:
> > On Jun 16 02:16, Mark Geisert wrote:
> > > I'm just putting a flag down on this new (to me) territory. If somebody
> > > else has claimed this project already, let me know and I'll shove off.
> > No, please. Just keep on working on that. If you manage to get jemalloc
> > working and replacing dlmalloc, this would be really great.
> > > It wasn't much trouble to build a jemalloc.lib and statically link it into
> > > the Cygwin DLL when the latter is built. I'm still learning which jemalloc
> > > configure options are required in order to get complete test coverage and to
> > > initialize properly within cygwin1.dll.
> > >
> > > I'm currently using the "supply your own malloc" mechanism provided by
> > > Cygwin's malloc_wrapper.cc to overlay the usual dlmalloc-sourced functions
> > > with replacements from jemalloc. I suspect there will be allocation
> > > collisions ahead...
> I've had to rethink the above a bit.
> > The real problem here is this:
> > __malloc_lock ();
> > dl_foo_function ();
> > __malloc_unlock ();
> > This locking is what makes our dlmalloc even slower in multi-threaded
> > scenarios because it disallows using malloc/free calls concurrently.
> > If you get jemalloc working, it would be nice in itself, but the main
> > improvement would be the ability to get rid of these __malloc_lock/
> > __malloc_unlock brackets.
> Thanks for reminding me of that aspect of Cygwin's current malloc. The
> malloc implementation has seemed to be bulletproof for many years so I guess
> the function-level locking is the only drawback of note?
Not quite. It's bad enough, given how much this slows down multi-threaded
...the big problem are dependencies on malloc during Cygwin startup,
especially in fork/exec, so the real challenge is to get the new malloc
still let Cygwin processes start up correctly first time and especially
in fork/exec situations, and to make sure the malloc bookkeeping
These malloc dependencies sometimes crop up in the weirdest situations,
so that's something to look out for. For instance, using pthread
functions may call malloc as well. If a problem can be solved by
changing another part of Cygwin, don't hesitate to discuss this!
> I've found that jemalloc would add 500kB to cygwin1.dll and it also seems
> difficult to get working, at first blush at least.
OTOH you leave dlmalloc behind, so that's 280kB less again.
> I've switched to a
> plug-in sort of implementation that allows one to choose among several
> malloc packages: "original", dlmalloc (w/ internal locking), ptmalloc,
> nedalloc, jemalloc, and a Windows Heap wrapper. Perhaps tcmalloc in the
> future. One sets an environment variable CYGMALLOC=<name> before launching
> a program and that malloc implementation is used. This should make testing
> and benchmarking the various choices possible. I don't expect big
> improvements in individual programs (unless they are stress testing), but
> something like a large configure or build should give more useful data.
In the end, we should settle for a single malloc implementation, though.
It doesn't really matter if it's jemalloc, ptmalloc, xymalloc. Almost
all other modern mallocs are faster and better suited for multi-threading
than dlmalloc, *especially* if the above locks can go away.
The only danger here is this: If you manage to get dlmalloc replaced
reliably, you *will* get a pink plush hippo!
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