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 -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
