Re: Memory sizing - DBA question
Date: 1996/04/07
Message-ID: <4k8vbh$rng_at_inet-nntp-gw-1.us.oracle.com>#1/1
utrankar_at_nyc.pipeline.com (Sameer Utrankar) wrote:
}
} >My questions are :
} >
} >1. All oracle background processes share the same memory ?
Yes and no. All background processes share the same text segment and shared memory segment(s). Each background process has a private data segment, which belongs to it, and it alone.
} >2. Shadow processes also share the same memory as background proceses
Shadow processes share the same text segment and shared memory
segment(s) as the background processes. Each shadow process also
has a private data segment.
} >? Is that why shared memory of shadow process is assumed to be 0 ?
It's best to think of shared memory as being associated with an *instance*, rather than with any process. The shared memory gets allocated, once, when the instance starts up, and gets shared among all processes accessing the instance.
} >3. When you run a tool - say sqlplus - shared and private memory of
} >it is considered only once - regardless of how many users are running
} >the same tool - why ?
When you run a foreground tool, such as sql*plus, memory is allocated for the text segment and the data segment. The memory allocated for the text segment is shared among all processes executing that particular executable image; each process allocates memory for its own data segment.
} >4. I always thought in any computer system, memory is always used by
} >one or more programs.
This is not correct.
} > SGA does not figure anywhere in this calculation
} >(and is added later separately) - what does that mean ? Which
} >process(es) owns SGA ? All background processes ? None ? Why doesn't
} >it figure into memory requirement of background processes ?
See above about shared memory allocated to the instance, not a process.
} >
} >On HP machine, using Glance Plus, when I look at memory usage of a
} >process, it shows RSS and VSS parts (Resident and Virtual) and VSS is
} >always more than RSS. Does VSS part include RSS ? Which one do I look
} >at for doing the caculations, similar to tuning book ? VSS or RSS ?
} >VSS figures seem to be more close to the book for sqlplus.
Unix is a virtual memory operating system. This means that memory allocated to a process may be physical memory (RAM), virtual memory (swap space), or a combination of the two. RSS is the amount of physical memory allocated to the process. VSS is the sum of the RSS and any memory pages paged out to the swap space, so it is necessarily larger than RSS.
To greatly over-simplify, you use the VSS size and the RSS size for sizing two different things: the VSS to size the swap space, and the RSS to size the RAM.
} >----------
} >utrankar_at_nyc.pipline.com (Sameer Utrankar)
Paul Zola Technical Specialist World-Wide Technical Support Development Tools ============================================================================== Computers possess the truly profound stupidity of the inanimate. - B. Sterling Disclaimer: Opinions and statements are mine, and do not necessarily reflect the opinions of Oracle Corporation.Received on Sun Apr 07 1996 - 00:00:00 CEST