Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1264774

Re: [PATCH 0/2] "big hammer" for DAX msync/fsync correctness

Show all headers | View raw

On Sat, 7 Nov 2015, Dan Williams wrote:
> Thanks for that explanation.  Peter had alluded to it at KS, but I
> indeed did not know that it was as horrible as milliseconds of
> latency, hmm...

Yes, I was pretty surprised as well. But even if it's just in the
hundreds of microseconds it can be too much for latency sensitive
applications.
 
> One other mitigation that follows on with Dave's plan of per-inode DAX
> control, is to also track when an inode has a writable DAX mmap
> established.  With that we could have a REQ_DAX flag to augment
> REQ_FLUSH to potentially reduce committing violence on the cache.  In
> an earlier thread I also recall an idea to have an mmap flag that an
> app can use to say "yes, I'm doing a writable DAX mapping, but I'm
> taking care of the cache myself".  We could track innocent cpus, but
> I'm thinking that would be a core change to write-protect pages when a
> thread migrates?  In general I feel there's a limit for how much
> hardware workaround is reasonable to do in the core kernel vs waiting
> for the platform to offer better options...

One thing vs. the mmaps: We exactly know which CPUs are involved in
that mapping. We know that from the TLB management. So we probably can
make use of that knowledge.

> Sorry if I'm being a bit punchy, but I'm still feeling like I need to
> defend the notion that DAX may just need to be turned off in some
> situations.

That's fine, if there is no reasonable way around it. It just needs to
be documented so people won't be surprised.

Thanks,

	tglx

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Back to linux.kernel | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Re: [PATCH 0/2] "big hammer" for DAX msync/fsync correctness Dan Williams <dan.j.williams@intel.com> - 2015-11-06 01:00 +0100
  Re: [PATCH 0/2] "big hammer" for DAX msync/fsync correctness Thomas Gleixner <tglx@linutronix.de> - 2015-11-06 09:10 +0100
    Re: [PATCH 0/2] "big hammer" for DAX msync/fsync correctness Dan Williams <dan.j.williams@intel.com> - 2015-11-06 17:10 +0100
      Re: [PATCH 0/2] "big hammer" for DAX msync/fsync correctness Thomas Gleixner <tglx@linutronix.de> - 2015-11-06 18:40 +0100
        Re: [PATCH 0/2] "big hammer" for DAX msync/fsync correctness Dan Williams <dan.j.williams@intel.com> - 2015-11-07 00:20 +0100
          Re: [PATCH 0/2] "big hammer" for DAX msync/fsync correctness "H. Peter Anvin" <hpa@zytor.com> - 2015-11-07 02:00 +0100
            Re: [PATCH 0/2] "big hammer" for DAX msync/fsync correctness Thomas Gleixner <tglx@linutronix.de> - 2015-11-07 08:00 +0100
              Re: [PATCH 0/2] "big hammer" for DAX msync/fsync correctness Dan Williams <dan.j.williams@intel.com> - 2015-11-07 09:20 +0100
                Re: [PATCH 0/2] "big hammer" for DAX msync/fsync correctness Thomas Gleixner <tglx@linutronix.de> - 2015-11-07 09:50 +0100
                Re: [PATCH 0/2] "big hammer" for DAX msync/fsync correctness Dan Williams <dan.j.williams@intel.com> - 2015-11-07 10:10 +0100
                Re: [PATCH 0/2] "big hammer" for DAX msync/fsync correctness Thomas Gleixner <tglx@linutronix.de> - 2015-11-07 10:30 +0100
  Re: [PATCH 0/2] "big hammer" for DAX msync/fsync correctness "H. Peter Anvin" <hpa@zytor.com> - 2015-11-06 21:30 +0100

csiph-web