On Thu, Aug 8, 2013 at 3:18 AM, Jan Kara [off-list ref] wrote:

On Wed 07-08-13 11:00:52, Andy Lutomirski wrote:

quoted

On Wed, Aug 7, 2013 at 10:40 AM, Dave Hansen [off-list ref] wrote:

quoted

On 08/07/2013 06:40 AM, Jan Kara wrote:

quoted

  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.

True.  The nasty part will be all the code that assumes that the acts
of un-write-protecting and dirtying are the same thing, for example
__block_write_begin, which is why I don't really believe in my
willwrite patches...

quoted

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?

I haven't measured it yet, because I suspect that my patches are
rather buggy in their current form.  But the idea is that fallocate
will do the heavy lifting and give me a nice contiguous allocation,
and the MADV_WILLNEED call will take about as long as the first write
fault would have taken.  Then the first write fault after
MADV_WILLNEED will take about as long as the second write fault would
have taken without it.


--Andy

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