Re: fsync fixes for 2.4

[Andrea Arcangeli]

On Fri, Jul 12, 2002 at 02:52:11PM -0700, Griffiths, Richard A wrote:
> Mark is off climbing Mt. Hood, so he asked me to post the data on the fsync
> patch.
> It appears from these results that there is no appreciable improvement using
> the
> fsync patch - there is a slight loss of top end on 4 adapters 1 drive. 

that's very much expected, as said with my new design by adding an
additional pass (third pass), I could remove the slight loss that I
expected from the simple patch that puts wait_on_buffer right in the
first pass.

I mentioned this in my first email of the thread, so it looks all right.
For a rc2 the slight loss sounds like the simplest approch.

If you care about it, with my new fsync accounting design we can fix it,
just let me know if you're interested about it. Personally I'm pretty
much fine with it this way too, as said in the first email if we block
it's likely bdflush is pumping the queue for us. the slowdown is most
probably due too early unplug of the queue generated by the blocking
points.

as for the scaling with async flushes to multiple devices, 2.4 has a
single flushing thread, 2.5 as Andrew said (partly) fixes this as he
explained me at OLS, with multiple pdflush. The only issue I seen in his
design is that he works based on superblocks, so if a filesystem is on
top of a lvm backed by a dozen of different harddisks, only one pdflush
will pump on those dozen physical request queues, because the first
pdflush entering the superblock will forbid other pdflush to work on the
same superblock too. So the first physical queue that is full, will
forbid pdflush to push more dirty pages to the other possibly empty
physical queues.

without lvm or raid that doesn't matter thoguh, nor it matters with
hardware raid.

Andrea