I am on the amd64 arch, on intel c2d.
I can't understand something simple :
A segment descriptor is a the representation of a process (code segment, data, stack..)
How is it that we are stil limited to 4Gb on the amd64 architecture (that is the limit field in the descriptors).
Should it be 64bit limit and not a 32bit one ?
Related
I recently started working on my own operating system. I am following jsandler18‘s awesome tutorial and making changes as I go to allow it to run on the raspberry pi 4.
Sadly, jsandler18 stopped updating the tutorial before he had finished the page on virtual memory. I read through some other sources, and found a little problem: The ARM l1 address translation table divides the computers RAM into 1-MB blocks. The problem here is that it only allows up to 4096 entries, or 4GB of virtual ram.
Is there some way I can use the ARM MMU to translate more than 4GB of virtual memory?
The tutorial being referenced appears to be executing in ARMV7, which can be thought of as 32-bit ARM. This is roughly equivalent to running in 32-bit PAE mode in X86. Thus using this example it is not possible to to use more that 4GB of virtual memory.
ARMV8 (or AARCH64) supports 64-bit virtual addresses, and would allow mapping more that 4GB of virtual memory.
Switching into ARMV8 is done by switching Exception levels, which are usually denoted as EL0, EL1, EL2 and EL3. The one challenge you could run into is that once you enter AARCH32 mode, you can not go to a lower exception level and switch to AARCH64. For example going from EL1 64-bit -> EL0 32-bit is supported, but going EL1 32-bit -> EL0 64-bit is not. This could pose a challenge if the firmware handing execution off to your OS is in AARCH32 mode.
In a Computer architecture book, I studied that Pentium IV has 64 bits bus width and addressable memory of 64 gigabytes. Does it mean that a computer running Pentium IV can support 64GB RAM, if supported by the OS?
This is so-called Physical Address Extension, which was supported by both Intel and AMD processors. Yes, with proper OS support 64Gb of RAM could be addressed. PLease read https://en.wikipedia.org/wiki/Physical_Address_Extension for more information
I am using qemu on Fedora and I find that qemu does not support multi-core. When I use the parameter smp and set cores=2, it will tell me that:
mu-system-riscv: Number of SMP CPUs requested (2) exceeds max CPUs supported by machine 'riscv'
In general, QEMU can support multicore guests, yes. However the number of cores supported depends on the particular board (machine) model you're using. The error message is telling you that the 'riscv' machine you've asked for only supports one CPU.
(In TCG emulation at the moment multicore guests won't be any faster than a single core guest because we don't use all the host cores; this should change in QEMU 2.9 for at least some host/guest combinations when multithreaded TCG support lands. KVM supports multicore guests with no problems.)
I have installed Memcached along with OPCache Zend. And I wonder why it is using so low memory. Does this stat's are good ?.
I have configured about 20sites to use it. Wordpress, Magento, Joomla
My Server spec are :
Intel Xeon X3440 Quad-Core
Memory 16GB
x2 1500 GB SATA II-HDD 7.200 rpm
CentOS 5 - Plesk 10 - RAID 1
This is output from Memcached Admin window:
This is my configuration from memcache.ini
extension=memcache.so
session.save_handler=memcache
session.save_path="tcp://localhost:11211?persistent=1&weight=1&timeout=1&retry_interval=15"
This is my configuration from memcached in sysconfig:
PORT="11211"
USER="memcached"
MAXCONN="512"
CACHESIZE="32"
OPTIONS="-f 1.5 -I 2800 -l 127.0.0.1"
It is using a very low amount of memory because you're CACHESIZE variable in /etc/sysconfig/memcached is 32, it will never use more than 32mb. Try increasing it to 64 and you're server might perform slightly faster (depending on your coding implementation of course).
as here mention, mongodb has limitation of datasize to 2GB in 32-bit machine with one single mongod instance. But I wonder 32-bit machine has 4GB addressable space in theory, and mongod can use this 4GB instead of 2GB for virtual memory usage. So why the answer is 2GB, not 4GB?
4Gb of addressable space is not the same as the memory space available for memory-mapped files opened by user applications. Some of the addressable space is reserved for the O/S kernel and memory-mapped devices such as video cards.
For example, 32-bit Windows limits user mode (and thus memory-mapped files) to ~2Gb RAM and total system RAM to ~3.5Gb.
For more reading, see:
Coding Horror: Dude, Where's My 4 Gigabytes of RAM?
MSDN: Managing Memory-Mapped Files
MSDN: Memory-Mapped Files
The majority of modern desktop and server environments starting moving to 64-bit almost a decade ago (see 64-bit operating system timeline on Wikipedia) so this isn't a limit that practically affects deployment.
You would only want to use 32-bit MongoDB in a development environment with limited data.
32-bit MongoDB processes are limited to about 2 gb of data. This has come as a surprise to a lot of people who are used to not having to worry about that. The reason for this is that the MongoDB storage engine uses memory-mapped files for performance.
By not supporting more than 2gb on 32-bit, we’ve been able to keep our code much simpler and cleaner. This greatly reduces the number of bugs, and reduces the time that we need to release a 1.0 product.
http://blog.mongodb.org/post/137788967/32-bit-limitations