_gc_fusion_compression

We know that database blocks are transferred between the nodes through the interconnect, aka cache fusion traffic. Common misconception is that packet transfer size is always database block size for block transfer (Of course, messages are smaller in size). That’s not entirely true. There is an optimization in the cache fusion code to reduce the packet size (and so reduces the bits transferred over the private network). Don’t confuse this note with Jumbo frames and MTU size, this note is independent of MTU setting.

In a nutshell, if free space in a block exceeds a threshold (_gc_fusion_compression) then instead of sending the whole block, LMS sends a smaller packet, reducing private network traffic bits. Let me give an example to illustrate my point. Let’s say that the database block size is 8192 and a block to be transferred is a recently NEWed block, say, with 4000 bytes of free space. Transfer of this block over the interconnect from one node to another node in the cluster will result in a packet size of ~4200 bytes. Transfer of bytes representing free space can be avoided completely, just a symbolic notation of free space begin offset and free space end offset is good enough to reconstruct the block in the receiving side without any loss of data.This optimization makes sense as there is no need to clog the network unnecessarily.

My COLLABORATE 12-IOUG sessions

If you are attending Collaborate 2012, you might be interested in my content-rich sessions below :

Session Number: 326
Session Title: SCAN, VIP, HAIP, and other RAC acronyms
Session Date/Time/Room: Tue, Apr 24, 2012 (10:45 AM – 11:45 AM) : Surf C

Session Number: 327
Session Title: Internals and Performance Boot Camp: Truss, pstack, pmap, and more
Session Date/Time/Room: Wed, Apr 25, 2012 (03:00 PM – 04:00 PM) : Palm A

Hope to see you there!

Update: I am uploading presentation files. Presentations are much more recent than the document 🙂

pstack_truss_etc
2012_327_Riyaj_pstack_truss_doc
SCAN_VIP_HAIP_etc
2012_326_Riyaj_scan_vip_haip_doc

Thanks for attending!

gc buffer busy acquire vs release

Last week (March 2012), I was conducting Advanced RAC Training online. During the class, I was recreating a ‘gc buffer busy’ waits to explain the concepts and methods to troubleshoot the issue.

Definitions

Let’s define these events first. Event ‘gc buffer busy’ event means that a session is trying to access a buffer,but there is an open request for Global cache lock for that block already, and so, the session must wait for the GC lock request to complete before proceeding. This wait is instrumented as ‘gc buffer busy’ event.

From 11g onwards, this wait event is split in to ‘gc buffer busy acquire’ and ‘gc buffer busy release’. An attendee asked me to show the differentiation between these two wait events. Fortunately, we had a problem with LGWR writes and we were able to inspect the waits with much clarity during the class.

Remember that Global cache enqueues are considered to be owned by an instance. From 11g onwards, gc buffer busy event differentiated between two cases:

1. If existing GC open request originated from the local instance, then current session will wait for ‘gc buffer busy acquire’. Essentially, current process is waiting for another process in the local instance to acquire GC lock, on behalf of the local instance. Once GC lock is acquired, current process can access that buffer without additional GC processing (if the lock is acquired in a compatible mode).
2. If existing GC open request originated from a remote instance, then current session will wait for ‘gc buffer busy release’ event. In this case session is waiting for another remote session (hence another instance) to release the GC lock, so that local instance can acquire buffer.

Example

Following output should show the differentiation with much clarity.