Re: [HACKERS] Faster CREATE DATABASE by delaying fsync (was 8.4.1 ubuntu karmic slow createdb)

Andres,
Great job. Looking through the emails and thinking about why this works I
think this patch should significantly speedup 8.4 on most any file
system(obviously some more than others) unless the system has significantly
reduced memory or a slow single core. On a Celeron with 256 memory I suspect
it'll crash out or just hit the swap and be a worse bottleneck. Anyone
have something like this to test on?
-Michael

On Mon, Dec 28, 2009 at 9:05 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:

> On Tuesday 29 December 2009 01:46:21 Greg Smith wrote:
> > Andres Freund wrote:
> > > As I said the real benefit only occurred after adding posix_fadvise(..,
> > > FADV_DONTNEED) which is somewhat plausible, because i.e. the directory
> > > entries don't need to get scheduled for every file and because the
> kernel
> > > can reorder a whole directory nearly sequentially. Without the advice
> it
> > > the kernel doesn't know in time that it should write that data back and
> > > it wont do it for 5 seconds by default on linux or such...
> > It would be interesting to graph the "Dirty" and "Writeback" figures in
> > /proc/meminfo over time with and without this patch in place. That
> > should make it obvious what the kernel is doing differently in the two
> > cases.
> I did some analysis using blktrace (usefull tool btw) and the results show
> that
> the io pattern is *significantly* different.
>
> For one with the direct fsyncing nearly no hardware queuing is used and for
> another nearly no requests are merged on software side.
>
> Short stats:
>
> OLD:
>
> Total (8,0):
> Reads Queued: 2, 8KiB Writes Queued: 7854,
> 29672KiB
> Read Dispatches: 2, 8KiB Write Dispatches: 1926,
> 29672KiB
> Reads Requeued: 0 Writes Requeued: 0
> Reads Completed: 2, 8KiB Writes Completed: 2362,
> 29672KiB
> Read Merges: 0, 0KiB Write Merges: 5492,
> 21968KiB
> PC Reads Queued: 0, 0KiB PC Writes Queued: 0,
> 0KiB
> PC Read Disp.: 436, 0KiB PC Write Disp.: 0,
> 0KiB
> PC Reads Req.: 0 PC Writes Req.: 0
> PC Reads Compl.: 0 PC Writes Compl.: 2362
> IO unplugs: 2395 Timer unplugs: 557
>
>
> New:
>
> Total (8,0):
> Reads Queued: 0, 0KiB Writes Queued: 1716,
> 5960KiB
> Read Dispatches: 0, 0KiB Write Dispatches: 324,
> 5960KiB
> Reads Requeued: 0 Writes Requeued: 0
> Reads Completed: 0, 0KiB Writes Completed: 550,
> 5960KiB
> Read Merges: 0, 0KiB Write Merges: 1166,
> 4664KiB
> PC Reads Queued: 0, 0KiB PC Writes Queued: 0,
> 0KiB
> PC Read Disp.: 226, 0KiB PC Write Disp.: 0,
> 0KiB
> PC Reads Req.: 0 PC Writes Req.: 0
> PC Reads Compl.: 0 PC Writes Compl.: 550
> IO unplugs: 503 Timer unplugs: 30
>
>
> Andres
>