Dbwr consuming high CPU after enabling DirectIO

Question

Hi,
DBWR is consuming high CPU. After enabling DirectIO on Solaris SPARC 10, dbwr is eating away almost 1 CPU on a v440 machine i.e. 19% throughout the day. Neither of "buffer busy waits" or "write complete waits" or "free buffer waits" are in the top 5 wait events, which, to me, means that there is no buffer contention.
What I understand is that after enabling DirectIO, it takes longer for the IO to complete because pre-DirectIO it would return from the file system cache whereas now it has to return from the disk (and I do see at the OS level that IO has become slow), but should that result in dbwr consuming more CPU?
Infact after enabling DirectIO, IO has become very slow which is another problem and as a result log file writes have also become slow which is a 3rd problem. btw, I am aware that if there were many FTS, then DirectIO can make the system slow but there are no FTS in my case. Also, that SGA should be increased after enabling DirectIO, which has also been done.
Thanks

Answer 1

user12022918 wrote:
DBWR is consuming high CPU. After enabling DirectIO on Solaris SPARC 10, dbwr is eating away almost 1 CPU on a v440 machine i.e. 19% throughout the day. 19% is less than 1/5th of a CPU. Or are you referring to a 100% being all 4 CPUs?
What I understand is that after enabling DirectIO, it takes longer for the IO to complete because pre-DirectIO it would return from the file system cache whereas now it has to return from the disk Incorrect. See directio for details.
Yes, removing the file system cache from the I/O layer for a device can reduce I/O performance if the caller does not perform its own caching. However, direct I/O will eliminate the system cache overheads (and associated CPU resources needed) from a caller (like Oracle) that implements its own sophisticated buffer cache.
Direct I/O should therefore increase Oracle I/O performance and decrease resource footprint as it eliminates the need for the kernel to maintain a cache for that device.
Infact after enabling DirectIO, IO has become very slow which is another problem and as a result log file writes have also become slow which is a 3rd problem. Direct I/O, as per the Sun docs, is an advisory call. It may not place that device in direct I/O modes. It may result in partial direct I/O. So you need to make sure exactly what happens and how successful (partial or complete) this setting was.
btw, I am aware that if there were many FTS, then DirectIO can make the system slow but there are no FTS in my case. FTS (multi block reads/large sequential reads) is slower? This is contrary to Sun's docs that state:
Large sequential I/O generally performs best with DIRECTIO_ON, except when a file is sparse or is being extended and is opened with O_SYNC or O_DSYNC.