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The way I understand the notion of a 'process' is that it is a running instance of an executable program.The exe is in the secondary memory and the running instance of it is in the RAM. If this understanding is right, I would like to know what is really meant by this abstract description: 'Dividing a process into 'pages' and running some of the pages in RAM and keeping the rest in secondary memory for swapping when needed'? The question here is in the context of virtual memory.
Adding a 'programming' context to the question, following suggestions from moderators:
Say I write a small program to list the numbers from 1 to 100 (or) to print 'Hello world' (or) some desktop utility to scan a text file and print the words in the file one by one within the desktop window. Considering the final executable I have, once these programs are compiled and linked, how can the executable be 'divided' and run in parts in RAM when I run the executable? How shall I grasp and visualise the idea of what 'should be' in RAM at a point in time and what 'should not'?
You have it (the division) right there, in the virtual to physical address translation. The virtual address space is split into blocks of one or several kilobytes (typically, all of the same size), each of which can be associated with a chunk (page) of physical memory of the same size.
Those parts of the executable (or process) that haven't been used yet or haven't been used recently need not to be copied into the physical memory from the disk and so the respective portions of the virtual address space may not be associated with physical memory either. When the system becomes low on free physical memory, it may repurpose some pages, saving their contents to the disk if necessary (or not saving, if they contain read-only data/code).
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From the available information I understood that setting disk cache size in selenium will help in faster loading of the web pages, when we are doing the scraping or anything on single website. But my question is what good will it do if we set the disk cache size while dealing with multiple websites?
Or is it in fact bad to set disk cache size? When scraping multiple web pages i.e. in a way the websites can trace that we are scraping?
Disk Cache is a cache memory that is used to speed up the process of storing and accessing data from the host machine hard disk. It enables faster processing during reading/writing, issuing commands and other I/O process between the hard disk, the memory and computing components. A disk cache is also referred to as a disk buffer or cache buffer.
Chromium disk cache
The disk cache stores resources fetched from the web so that they can be accessed quickly at a latter time if needed. The main characteristics are:
The cache should not grow unbounded so there must be an algorithm for deciding when to remove old entries.
While it is not critical to lose some data from the cache, having to discard the whole cache should be minimized. The current design should be able to gracefully handle application crashes, no matter what is going on at that time, only discarding the resources that were open at that time. However, if the whole computer crashes while we are updating the cache, everything on the cache probably will be discarded.
Access to previously stored data should be reasonably efficient, and it should be possible to use synchronous or asynchronous operations.
We should be able to avoid conflicts that prevent us from storing two given resources simultaneously. In other words, the design should avoid cache trashing.
It should be possible to remove a given entry from the cache, and keep working with a given entry while at the same time making it inaccessible to other requests (as if it was never stored).
The cache should not be using explicit multithread synchronization because it will always be called from the same thread. However, callbacks should avoid reentrancy problems so they must be issued through the thread's message loop.
Conclusion
To conclude, by default google-chrome will be configured with the default value for the diskcache which users can configure as per their respective usecases.
Changing Chrome Cache size on Windows 10
There is only one method that can be used to set and limit Google Chrome’s cache size.
Launch Google Chrome.
Right-click on the icon for Google Chrome on the taskbar and again right-click on the entry labeled as Google Chrome.
Now click on Properties. It will open the Google Chrome Properties window.
Navigate to the tab labeled as Shortcut.
In the field called Target, type in the following after the whole address:
-disk-cache-size-<size in bytes>
As an example, to configure it as -disk-cache-size-2147483648:
"C:\Program Files (x86)\Google\Chrome\Application\chrome.exe" -disk-cache-size-2147483648
Here 2147483648 is the size of the cache in bytes which is equal to 2 Gigabytes.
Click on Apply and then click on OK for the limit to be set.
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Is there a free, fast way to recover the contents of an sd card? I have an sd card that originally belonged to a camera, with videos and photos on it. Recently, the camera no longer can take pictures, and the sd card looks like this on a computer.
The files and folders are replaced with random symbols, and are impossible to open.
I've tried multiple computers, and a software that didn't let me recover the images without paying. Is there a way to do it?
I recommend using a data ripper - it looks up file magic bytes, and copies the whole files into separate location. If your camera has corrupted the SD card when plugged out and in, I suppose you'll be able to recover min. 70% of data, but don't assume this, as it is computing black magic. There are many free data rippers.
Anyways, this question belongs to your camera or SD card vendor's support tracker.
Piriform Recuva is a free option:
https://www.piriform.com/recuva
The following was copied from the Piriform website Recuva documentation at the time of this writing (source):
What it can and cant do
Recuva can:
Scan through your hard drives, memory cards, and USB sticks to find files and folders you've deleted.
Tell you in advance how likely it is that your file(s) can be recovered.
Recover files that Windows can't (see Problems with Windows and file deletion)
Securely delete a file you may have previously deleted.
Recover emails you deleted 'permanently' from Microsoft Outlook Express, Mozilla Thunderbird, or Windows Live Mail.
Recover files from your iPod, iPod Nano, or iPod Shuffle (iPod Touch and iPhone not supported at this time). Recuva will even recover songs with Apple's FairPlay DRM.
Recover Canon RAW (.CRW) format image files.
Recover files from NTFS, FAT, and exFAT-formatted drives.
Bring your files back!
Recuva cannot:
Recover all files. Yes, as great as Recuva is it won't work all the time. Sometimes Windows has overwritten the area where the file used to be, or sometimes the file is too corrupted to recover.
Recover files you've deleted securely. For example, if you've used our CCleaner software to delete files using the Secure option, they're gone for good.
Securely delete certain very small files that are held in the Master File Table (MFT) and files of zero-byte length.
Recuva is capable of recovering from NAS devices, however the drive needs to be connected directly to the machine via USB/IDE/SATA. Recuva is not capable of recovering data over a network.
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I am running Ubuntu on my physical machine; and VirtualBox to run various virtual ones on top.
Most of the time, I was doing "straight forward" installations; but today I wanted to be "smart" and checked out the partitions that the fedora or ubuntu installers will create on my virtual disks.
And sure, when going for the defaults, some GBs of my virtual disks will be used as "swap space".
Now I am wondering: assuming that I have plenty of physical memory (so I can assign 6 or 8 GB to a VM) - is there any sense in dedicated swap space for a a virtual machine?
This is answered at ServerFault:
TL;DR: use swap: 1. avoids out of memory error, 2. guest OS is better at memory management
Ignoring the fact that people are dealing with OS specific reasons I
have two reasons why it's a bad idea to not run with a swap
partition/file.
If you have 1.5 GB of RAM allocated to a VM with no space file/partition and it wants to use 1.5 GB + 1 MB it will report an out
of memory error. With the swap space it will be able to swap data out
of active memory and onto the disk.
The guest OS does a much better job of memory management than the host. This is why technology like memory ballooning exists because the
Host can make educated guesses on what memory isn't needed right now
but the guest knows at a much more intelligent level (this keeps OS
memory from being swapped out which could kill your performance).
Swap partitions are used to free your physical memory when it goes out of space. In modern day machines, with plenty of memory, it depends on the type of applications you would be running. If you are planning to run such memory intensive programs like video editors, high end games or something of that sort, virtual memory or swap space is an asset. But if it is not the case then you are safe to avoid swap space, provided you have enough memory. But it is safe to have a fallback.
That depends on what programs you are running on your host system along with the virtual machine, or what programs you are running within the virtual machine.
The only upper bound on memory that software can consume is the total memory (physical or virtual) available to it. There are plenty of programs that require large amounts of memory when behaving normally, and plenty of circumstances that cause a program to consume large amounts of memory (e.g. loading of input files). There are also plenty of faulty programs that unintentionally consume large amounts of memory.
You can often get an idea by examining requirements or recommendations (e.g. memory and drive space) of the programs you run. Failing that, try it out.
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Suppose we have a simple processor, could be an embedded system, with one system bus, for the sake of the argument, a 32bit bus.
Now, if we have a couple of Peripherals, one named PER0 for example, attached to the bus, we can do two things:
Allow it to have fixed-width access to the main bus, for example 8 bits, and that way PER0 will always communicate with the bus in 8bit packages. This we can call static-width access.
Allow it to have options to choose how it will communicate with the
bus in terms of size of data by using signals with which it tells
the processor the mode of access it wants to use. For example, we
create two signals, A1 and A0, between the processor and PER0, whose
values will say:
00 - wait
01 - 8bit
10 - 16bit
11 - 32bit
and so the processor will know whether to send 8bit data to its
bus, or 32bit data, based on the values of A1, A0. This we can call
dynamic-width access to the bus.
Question:
In your experience, which of these two methods is preferred, and why? Also, in which cases should this be implemented? And finally, considering embedded systems, which method is more widely spread?
EDIT: I would like to expand on this topic, so I'm not asking for personal preferences, but for further information about these two methods, and their applications in computer systems. Therefore, I believe that this qualifies as a legitimate stackoverflow question.
Thanks!
There are multiple considerations. Naturally, the dynamic-width would allow better utilization of bandwidth in case you have multiple sizes in your transactions. On the other hand, if you transfer some 8 bytes, and then the next 8, you double the overhead compared to the baseline (transferring the full block in one go, assuming you can cache it until it fully consumed). So basically you need to know how well you can tell in advance which chunks you're going to need.
There's an interesting paper about the possibility of using such a dynamic sized transactions between the CPU and the DRAM:
Adaptive granularity memory systems: a tradeoff between storage efficiency and throughput
There you can see the conflict since it's very hard to tell which transactions you'll need in the future and whether bringing only partial data may cause a degradation. They went to the effort of implementing a predictor to try and speculate that. Note that this is applicable to you only if you're dealing with coherent memory.
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I'm curious to understand what could be the motivation behind the fine-grained detail of each virtual processor that the Windows 8 task manager seems to be focusing on.
Here's a screenshot (from here):
I know this setup could only exist in a non-standard, costly, important server environment (1TB RAM!), but what is the use of a heatmap? Or, setting processor affinity:
What I'm asking is, under what circumstances a developer would care if specific processor X is being used more than processor Y (instead of just knowing that a single non-multithreaded process is maxing out a core, which would be better shown as a process heatmap, instead of a processor heatmap), or care whether a process will use this or that processor (which I can't expect a human to guess better than an auto-balancing algorithm)?
In most cases, it doesn't matter, and the heatmap does nothing more than look cool.
Big servers, though, are different. Some processors have a "NUMA", or Non-Uniform Memory Access, architecture. In these cases, some processor cores are able to access some chunks of memory faster than other cores. In these cases, adjusting the process affinity to keep the process on the cores with faster memory access might prove useful. Also, if a processor has per-core caches (as many do), there might be a performance cost if a thread were to jump from one core to another. The Windows scheduler should do a good job avoiding switches like these, but I could imagine in some strange workloads you might need to force it.
These settings could also be useful if you want to limit the number of cores an application is using (say to keep some other cores free for another dedicated task.) It might also be useful if you're running a stress test and you are trying to determine if you have a bad CPU core. It also could work around BIOS/firmware bugs such as the bugs related to high-performance timers that plagued many multi-core CPUs from a few years back.
I can't give you a good use case for this heat map (except that it looks super awesome), but I can tell you a sad story about how we used CPU affinity to fix something.
We were automating some older version of MS Office to do some batch processing of Word documents and Word was occasionally crashing. After a while of troubleshooting and desperation, we tried setting Word process' affinity to just one CPU to reduce concurrency and hence reduce the likelihood of race conditions. It worked. Word stopped crashing.
One possible scenario would be a server that is running multiple VMs where each client is paying to have access to their VM.
The administrator may set the processor affinities so that each VM has guaranteed access to X number of cores (and would charge the client appropriately).
Now, suppose that the administrator notices that the cores assigned to ABC Company Inc.'s VMs are registering highly on the heatmap. This would be a perfect opportunity to upsell ABC Company Inc and get them to pay for more cores.
Both the administrator and ABC Company Inc win - the administrator makes more money, and ABC Company Inc experience better performance.
In this way, the heatmap can function as a Decision Support System which helps ABC Company Inc decide whether their needs merit more cores, and helps the administrator to target their advertising better to the their customers that can benefit.