October 1999
Intermediate to advanced
136 pages
4h 15m
English
Content preview from Oracle Internals: An IntroductionBecome an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
O’Reilly covers everything we've got, with content to help us build a world-class technology community, upgrade the capabilities and competencies of our teams, and improve overall team performance as well as their engagement.I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.
Chapter 3. Latches
There are numerous data structures in Oracle’s System Global Area (SGA) that need to be accessed concurrently by many different database processes. It is essential that only one process be able to modify any particular data structure at one time, and that the data structure cannot be modified while it is being inspected. Oracle makes sure this does not happen by protecting all SGA data structures with either locks or latches. (See Chapter 6, for a description of the contents of the SGA and other memory areas.)
Latches and Locks
Latches are the more restrictive mechanism, because they do not allow multiple processes to inspect the protected data structure at the same time—they provide for exclusive access only.[1] Locks allow for better concurrency, because they may be held in a shared mode when the data structure is simply being inspected.
Another significant difference between locks and latches is request queuing. Requests for locks are queued if necessary and serviced in order, whereas latches do not support request queuing. If a request to get a latch fails because the latch is busy, the process just continues to retry until it succeeds. So latch requests are not necessarily serviced in order.
Because a latch can only be held by one process at a time, and because there is no inherent concept of queuing, the latch data structure itself is very simple—essentially just a single location in memory representing the state of the latch. And because the latch data structure ...
Become an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
and much more.
Read now
Unlock full access
More than 5,000 organizations count on O’Reilly
O’Reilly covers everything we've got, with content to help us build a world-class technology community, upgrade the capabilities and competencies of our teams, and improve overall team performance as well as their engagement.Julian F.
I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.Amir M.
I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.