I don't see a clear direct connection between OS directory entry cache and
log file sync duration (when it looked like LGWR was idle). Of course
sometimes there are surprises and it makes sense in hindsight, but this
would be a pretty complex chain of unfortunate events then :)
1) Dentry cache size (and any slowness) would only affect system calls when
trying to find, open files etc. Once the file is open, dentries should not
be in play as further system calls refer to the file via their file
descriptor that points directly to the inode.
2) Or perhaps a large dentry cache just consumes lots of kernel (SLAB/SLUB
allocator) memory and when reaching some limit, *other* system calls that
also hope to quickly allocate a standard-sized chunk from SLAB, end up
waiting (or even trying to reclaim other kernel memory!)
You could systematically measure this by running pSnapper on LGWR and one
of the test FG session processes at the same time. You can sample just
those 2 PIDs at first, sampling everything will make pSnapper slower.
You can run something like this in a loop (change 123,456 to your PIDs of
interest) - and it would show if those processes were in some
uninterruptible sleep (D) when executing a semop system call.
sudo psn -a -G pid,syscall,wchan,kstack -p 123,456
P.S. For others benefit, if you haven't used the Linux Process Snapper yet,
you should! ;-)
Examples here: https://tanelpoder.com/psnapper/
--
Tanel Poder
https://tanelpoder.com/seminar
On Thu, Feb 6, 2020 at 5:21 PM Chris Stephens <cstephens16@xxxxxxxxx> wrote:
I received this update from Oracle:
Discussed this issue with a colleague who mentioned the following--- Bug
25962858 : LOG FILE SYNCS ON A RELATIVELY IDLE DATABASE relates to DENTRY
Closed as not a bug.The solution in that bug was to periodically clear the
DENTRY cache.To check the size issueslabtop --onceThis will show something
like thisscorreia-linux::oracle> slabtop --once
Active / Total Objects (% used) : 10136104 / 10227087 (99.1%)
Active / Total Slabs (% used) : 273801 / 273801 (100.0%)
Active / Total Caches (% used) : 78 / 108 (72.2%)
Active / Total Size (% used) : 1572176.58K / 1601572.34K (98.2%)
Minimum / Average / Maximum Object : 0.01K / 0.16K / 15.19KOBJS ACTIVE USE
OBJ SIZE SLABS OBJ/SLAB CACHE SIZE NAME
5516472 5516241 99% 0.10K 141448 39 565792K buffer_head
1422582 1421855 99% 0.19K 67742 21 270968K dentry
<<<<----------------------------
481664 474527 98% 0.03K 3763 128 15052K kmalloc-32To clear the dentry cache
you can issue.... a reboot would do the sameecho 2 > /proc/sys/vm/drop_caches
