On Tue, Mar 25, 2008 at 12:19:30PM -0700, Alfred Perlstein wrote: > * Ruslan Ermilov <[EMAIL PROTECTED]> [080325 11:37] wrote: > > On Tue, Mar 25, 2008 at 11:01:52AM -0700, Alfred Perlstein wrote: > > > I don't think this was thought out enough, there are times when you > > > would want to limit the total memory allocated to mbufs and avoid > > > deadlocks in low memory situations. > > > > > > Even the old allocator could have been trivially modified to block > > > forever upon exhaustion of the mbuf arena. > > > > > > The reason why the old allocator was not "fixed" to block forever > > > was to allow for recovery from low memory deadlocks. > > > > > > A lot of work went into making the system safe in the face of these > > > deadlocks and removing it "to clean up" due to a deficiency with > > > the current allocator and without understanding why it was there > > > in the first place is a mistake. > > > > > > This whole thing needs to be backed out. > > > > > Are you (or anyone else you know) planning to work on adding real > > support for M_TRYWAIT? > > I would like to eventually, I think because my place of work moved > from 4.x to 6.x recently it will become an issue for us and we will > need to track it down, this will likely fall on my lap. Presently > the uma panics the machine when exhaustion happens, something that > can be averted by capping mbuf space. > > I spoke to John Baldwin about it and he said "it would be nice" and > would fix a number of panics at his place. That said he seems to > think that this change is OK as we'll just re-add the NULL checks > later on. He doesn't seem to support the backout or not support > it, no idea. > > However I'm not OK with it, because we spent many cycles fixing all > of these and new code will likely just assume the old thing which > will cause it to need substantial refactoring (see NFS history) to > be fixed or re-fixed. > > That said I don't have immediate plans for it, but I see it as a > requirement again as the userbase of 6.x and beyond grows. > Yes, it'd be nice to have the semantics M_TRYWAIT originally supposed to provide, but 1) it never worked as planned, and 2) four years have passed since MBUMA, and the code has rotten: some of it treated M_TRYWAIT as M_WAIT, some as "try to wait" (sometimes mixed in the single file), and some newer code now uses UMA flags M_WAITOK/M_NOWAIT directly, as hinted in mbuf.h.
Cheers, -- Ruslan Ermilov [EMAIL PROTECTED] FreeBSD committer _______________________________________________ cvs-all@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/cvs-all To unsubscribe, send any mail to "[EMAIL PROTECTED]"
