Re: Does Oracle PGA take out memory out of SGA?

Mladen,

Do you have a recommendation regarding the use of semaphores?

I've read about the position of RH (RedHat) and LH (Left Hand storage) for mapping out the semaphore alphabet. Flapping my arms about with red and yellow colored flags is having no effect and I feel like a real concurrency killer.

I don't see how increasing the number of semaphores is going to help but as the flags wear the number of threads on them is becoming larger.

Happy 1 April.

Paul

On Thu, Apr 1, 2021, 00:10 Mladen Gogala <gogala.mladen_at_gmail.com> wrote:

> ORA-7445 is an Oracle bug, by definition. You should open a service
> request with Oracle Support. Other than that, shared memory is an operating
> system trick. Virtual memory is based on page tables. There is a table
> somewhere in the OS which contains virtual addresses and their
> translations. Translation can be an address of a physical page in RAM or
> can be a page in a page file. If two processes map the same page table
> into their address space, we talk of shared memory. Yes, there are some
> details about that, such mapping is performed by shmat system call and not
> by sbrk, malloc or calloc. Shared memory is used for SGA. You can see the
> shared memory on your system by using ipcs -m.
>
> PGA, on the other hand, is allocated from the process private memory. That
> means that PGA consists of segments which are only mapped to a single
> process. PGA is allocated using sbrk, malloc or calloc. If PGA is expanded
> beyond limits, it will eventually hit another segment or OS limit and
> produce SIGSEGV "segmentation violation" signal and produce ORA-04036. Note
> that ORA-4036 is NOT the same as ORA-7445. ORA-7445 is a bug which needs to
> be resolved by Oracle Support. If you want to learn the basics of the Linux
> operating system, I would recommend 2 books:
>
> 1. Understanding the Linux Kernel by Daniel Bovet
> 2. Modern Operating Systems by Andrew Tannenbaum
>
> Both books are available on Amazon. Of course, the unspoken prerequisite
> for both books is certain book by Kernighan & Ritchie which should be
> taught to K12 children (ANSI edition, of course). What you are asking is
> quite complex to explain. The usual answer is that PGA and SGA are "two
> different types of memory" but I don't find that answer satisfactory. In
> order to understand the memory handling on Linux, you would need to
> understand interrupts, paging, processes, page tables, address spaces and
> IPC (Inter-Process Communication). PGA is allocated from the process
> private address space, SGA is allocated from shared memory. Now what is
> private memory space, how does Linux manage it, what is a page fault and
> other questions are rather difficult to explain. Of course, you are the
> only person who can decide whether you need deep knowledge of the Linux or
> not.
> On 3/31/21 3:03 PM, Amit Saroha wrote:
>
> Dear All,
>
> Please help me understanding PGA and SGA.
>
> I was under the impression that PGA and Memory and SGA Memory are
> differently allocated. But, in a recent situation, we observed *ORA-7445
> [kghalf] *and my DBA is saying - out of a total of 256 GB system memory
> one session's PGA consumed everything and only 8 MB was left for other
> sessions. But, apart from one process, no other processes were impacted so
> I have doubt that - Oracle consumes SGA memory and Allocates it to PGA of a
> single process.
>
> Please help me understand.
>
> --
> Mladen Gogala
> Database Consultant
> Tel: (347) 321-1217https://dbwhisperer.wordpress.com
>
> -- http://www.freelists.org/webpage/oracle-l

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