Re: [RFC 0/3] Add madvise(..., MADV_WILLWRITE)

Jan Kara
2013-08-08T10:19:23Z

On Wed 07-08-13 11:00:52, Andy Lutomirski wrote:
> On Wed, Aug 7, 2013 at 10:40 AM, Dave Hansen <dave.hansen@intel.com> wrote:
> > On 08/07/2013 06:40 AM, Jan Kara wrote:
> >> One question before I look at the patches: Why don't you use fallocate()
> >> in your application? The functionality you require seems to be pretty
> >> similar to it - writing to an already allocated block is usually quick.
> >
> > One problem I've seen is that it still costs you a fault per-page to get
> > the PTEs in to a state where you can write to the memory. MADV_WILLNEED
> > will do readahead to get the page cache filled, but it still leaves the
> > pages unmapped. Those faults get expensive when you're trying to do a
> > couple hundred million of them all at once.
>
> I have grand plans to teach the kernel to use hardware dirty tracking
> so that (some?) pages can be left clean and writable for long periods
> of time. This will be hard.
Right that will be tough... Although with your application you could
require such pages to be mlocked and then I could imagine we would get away
at least from problems with dirty page accounting.

> Even so, the second write fault to a page tends to take only a few
> microseconds, while the first one often blocks in fs code.
So you wrote blocks are already preallocated with fallocate(). If you
also preload pages in memory with MADV_WILLNEED is there still big
difference between the first and subsequent write fault?

> (mmap_sem is a different story, but I see it as a separate issue.)
Yeah, agreed.

Honza
--
Jan Kara <jack@suse.cz>
SUSE Labs, CR