I'm making a "big" non-embedded image intended for simulation instead of real devices, and I keep hitting the error:
*** Maybe you need to increase the filesystem size (BR2_TARGET_ROOTFS_EXT2_SIZE)
and then I have to do a du on output/target to find out how big I have to make BR2_TARGET_ROOTFS_EXT2_SIZE.
Is there a way to automate this, or a decent workaround?
Some workarounds I'm considering:
put the big stuff under 9p: https://superuser.com/questions/628169/how-to-share-a-directory-with-the-host-without-networking-in-qemu
use CPIO and -initrd
http://lists.busybox.net/pipermail/buildroot/2018-March/215622.html
http://lists.busybox.net/pipermail/buildroot/2018-March/215636.html says that:
No, becaaue it is not reliable, see commit:
c6bca8cef fs/ext2: Remove support for auto-calculation of rootfs size
In the end, it does not make sense to do auto-calculation, because on an
embedded device, you have to now the layout and size of your storage.
So, you know what size you want your ext filesystem to be.
So it is fundamentally not possible / worth for Buildroot to do it reliably.
https://github.com/buildroot/buildroot/commit/c6bca8cef0310bc649240b451989457ce94a8358
I have then searched a bit further, and came across: https://unix.stackexchange.com/questions/353156/how-to-calculate-the-correct-size-of-a-loopback-device-filesystem-image-for-debo which suggests resize2fs -M + sparse files might be a possibility.
libguestfs can also minimize image sizes automatically as demonstrated at https://serverfault.com/questions/246835/convert-directory-to-qemu-kvm-virtual-disk-image/916697#916697 and is exposes a vfs-minimum-size function: http://libguestfs.org/guestfish.1.html#vfs-minimum-size
Related
I'm implementing a small PCI driver for academic purposes, and one thing I'm not clear about if we actually have to provide driver.conf? Different materials which I read (including http://blog.csdn.net/hotsolaris/article/details/1763716), say that for PCI the driver config file is optional, however in my case it seems that pci_config_setup() is successful only with driver.conf provided:
name="mydrv" parent="/pci#0,0/pci8086,2e11"
Then I do:
% add_drv -i 'pciXXXX,YY' mydrv
and it adds in the system with no warning or error messages.
So I assume that some properties of a PCI device can't be derived automatically by the system, e.g. parent bus?
I would appreciate if anybody could shed some light on this. Thanks.
If you look at a random selection of very small files under /kernel/drv for actual physical hardware, you'll see that they almost always only contain the line
ddi_forceattach=1;
Pseudo drivers will have a driver.conf(4) file which reflects their parentage in the system. I really recommend reading that manpage, it goes into good detail about what's required here.
Is there a way to limit the amount of time an evaluation is allowed to run? Or limit the amount of memory that MatLab is allowed to take up so it doesn't freeze my laptop?
Let's answer your questions one at a time:
Question #1 - Can I limit the amount of time MATLAB takes to execute a script?
As far as I know, this is not possible. If you want to do this, you would need a multi-threaded environment where one thread does the actual job while another thread keeps an eye on the timer... but even with that functionality, AFAIK, MATLAB does not have this supported. The only way to stop your script from running is if you punch Ctrl + C / Cmd + C. Depending on what is actually being executed... for example a MEX script or a LAPACK routine, or just a simple MATLAB script, it may work by just pushing it once... or you may have to mash the sequence like a maniac.
(Note: The above image was introduced to try and be funny. If you don't know where that image is from, it's from the movie Flashdance and one of the songs from the soundtrack is She's a maniac, where I've also provided a YouTube link to the song above.)
See this post for more details: How can I interrupt MATLAB when it gets really really busy?
Question #2 - Can we limit the amount of memory that MATLAB uses?
Yes you can. From what I have seen in your posts, you're using Windows. You can change this by changing the page size of the virtual memory that is used for your computer. Specifically, instead of allowing it to grow dynamically, you could set it to be a certain size and once MATLAB exhausts that, it'll give you an out-of-memory error rather than freezing your computer.
See this post from MathWorks forums for more insight:
http://www.mathworks.com/matlabcentral/answers/12695-put-a-limit-on-memory-matlab-uses
Also see this guide from MathWorks on how to handle out-of-memory errors:
http://www.mathworks.com/help/matlab/matlab_prog/resolving-out-of-memory-errors.html
Finally, take a look at this link on how to change / modify the page size of your computer via Windows:
http://windows.microsoft.com/en-ca/windows/change-virtual-memory-size#1TC=windows-7
I need to increase the FD_SETSIZE value from 1024 to 4096. I know it'd be better to use poll()/epoll() but I want to understand what are pros/cons. The main question is: have I to recompile glibc? I read several thread where the change of .h after changing FD_SETSIZE works recompiling only the user application. Reading the glibc code (and the kernel too), actually it seems to me that if I want to use select(), FD_* macro and so on, I have to recompile all because the size of fd_set is changed. At this point I have to recompile all not only my application because if in the system there is an another "common" application that uses select and friends, I could have problem. Am I right?
Technically, you do not have to recompile glibc. It would be sufficient to use come up with your own version of <sys/select.h> that has a larger fd_set_t, but is otherwise compatible. It will magically work because the select function receives the largest file descriptor (plus one), so it can figure out the set sizes. The other functions and macros are either inline or do not care about the actual set size.
It's still a bad idea, so you really should be using poll or epoll instead.
In the past, some libcs supported defining FD_SETSIZE before including <sys/select.h> to obtain a larger set size, but I don't think support for that was ever part of mainline glibc.
I'm looking at some slightly confused code that's attempted a platform abstraction of prefetch instructions, using various compiler builtins. It appears to be based on powerpc semantics initially, with Read and Write prefetch variations using dcbt and dcbtst respectively (both of these passing TH=0 in the new optional stream opcode).
On ia64 platforms we've got for read:
__lfetch(__lfhint_nt1, pTouch)
wherease for write:
__lfetch_excl(__lfhint_nt1, pTouch)
This (read vs. write prefetching) appears to match the powerpc semantics fairly well (with the exception that ia64 allows for a temporal hint).
Somewhat curiously the ia32/amd64 code in question is using
prefetchnta
Not
prefetchnt1
as it would if that code were to be consistent with the ia64 implementations (#ifdef variations of that in our code for our (still live) hpipf port and our now dead windows and linux ia64 ports).
Since we are building with the intel compiler I should be able to many of our ia32/amd64 platforms consistent by switching to the xmmintrin.h builtins:
_mm_prefetch( (char *)pTouch, _MM_HINT_NTA )
_mm_prefetch( (char *)pTouch, _MM_HINT_T1 )
... provided I can figure out what temporal hint should be used.
Questions:
Are there read vs. write ia32/amd64 prefetch instructions? I don't see any in the instruction set reference.
Would one of the nt1, nt2, nta temporal variations be preferred for read vs. write prefetching?
Any idea if there would have been a good reason to use the NTA temporal hint on ia32/amd64, yet T1 on ia64?
Are there read vs. write ia32/amd64 prefetch instructions? I don't see any in the instruction set reference.
Some systems support the prefetchw instructions for writes
Would one of the nt1, nt2, nta temporal variations be preferred for read vs. write prefetching?
If the line is exclusively used by the calling thread, it shouldn't matter how you bring the line, both reads and writes would be able to use it. The benefit for prefetchw mentioned above is that it will bring the line and give you ownership on it, which may take a while if the line was also used by another core. The hint level on the other hand is orthogonal with the MESI states, and only affects how long would the prefetched line survive. This matters if you prefetch long ahead of the actual access and don't want to prefetch to get lost in that duration, or alternatively - prefetch right before the access, and don't want the prefetches to thrash your cache too much.
Any idea if there would have been a good reason to use the NTA temporal hint on ia32/amd64, yet T1 on ia64?
Just speculating - perhaps the larger caches and aggressive memory BW are more vulnerable to bad prefetching and you'd want to reduce the impact through the non-temporal hint. Consider that your prefetcher is suddenly set loose to fetch anything it can, you'd end up swamped in junk prefetches that would through away lots of useful cachelines. The NTA hint makes them overrun each other, leaving the rest undamaged.
Of course this may also be just a bug, I can't tell for sure, only whoever developed the compiler, but it might make sense for the reason above.
The best resource I could find on x86 prefetching hint types was the good ol' article What Every Programmer Should Know About Memory.
For the most part on x86 there aren't different instructions for read and write prefetches. The exceptions seem to be those that are non-temporal aligned, where a write can bypass the cache but as far as I can tell, a read will always get cached.
It's going to be hard to backtrack through why the earlier code owners used one hint and not the other on a certain architecture. They could be making assumptions about how much cache is available on processors in that family, typical working set sizes for binaries there, long term control flow patterns, etc... and there's no telling how much any of those assumptions were backed up with good reasoning or data. From the limited background here I think you'd be justified in taking the approach that makes the most sense for the platform you're developing on now, regardless what was done on other platforms. This is especially true when you consider articles like this one, which is not the only context where I've heard that it's really, really hard to get any performance gain at all with software prefetches.
Are there any more details known up front, like typical cache miss ratios when using this code, or how much prefetches are expected to help?
I'm developing an operating system and rather than programming the kernel, I'm designing the kernel. This operating system is targeted at the x86 architecture and my target is for modern computers. The estimated number of required RAM is 256Mb or more.
What is a good size to make the stack for each thread run on the system? Should I try to design the system in such a way that the stack can be extended automatically if the maximum length is reached?
I think if I remember correctly that a page in RAM is 4k or 4096 bytes and that just doesn't seem like a lot to me. I can definitely see times, especially when using lots of recursion, that I would want to have more than 1000 integars in RAM at once. Now, the real solution would be to have the program doing this by using malloc and manage its own memory resources, but really I would like to know the user opinion on this.
Is 4k big enough for a stack with modern computer programs? Should the stack be bigger than that? Should the stack be auto-expanding to accommodate any types of sizes? I'm interested in this both from a practical developer's standpoint and a security standpoint.
Is 4k too big for a stack? Considering normal program execution, especially from the point of view of classes in C++, I notice that good source code tends to malloc/new the data it needs when classes are created, to minimize the data being thrown around in a function call.
What I haven't even gotten into is the size of the processor's cache memory. Ideally, I think the stack would reside in the cache to speed things up and I'm not sure if I need to achieve this, or if the processor can handle it for me. I was just planning on using regular boring old RAM for testing purposes. I can't decide. What are the options?
Stack size depends on what your threads are doing. My advice:
make the stack size a parameter at thread creation time (different threads will do different things, and hence will need different stack sizes)
provide a reasonable default for those who don't want to be bothered with specifying a stack size (4K appeals to the control freak in me, as it will cause the stack-profligate to, er, get the signal pretty quickly)
consider how you will detect and deal with stack overflow. Detection can be tricky. You can put guard pages--empty--at the ends of your stack, and that will generally work. But you are relying on the behavior of the Bad Thread not to leap over that moat and start polluting what lays beyond. Generally that won't happen...but then, that's what makes the really tough bugs tough. An airtight mechanism involves hacking your compiler to generate stack checking code. As for dealing with a stack overflow, you will need a dedicated stack somewhere else on which the offending thread (or its guardian angel, whoever you decide that is--you're the OS designer, after all) will run.
I would strongly recommend marking the ends of your stack with a distinctive pattern, so that when your threads run over the ends (and they always do), you can at least go in post-mortem and see that something did in fact run off its stack. A page of 0xDEADBEEF or something like that is handy.
By the way, x86 page sizes are generally 4k, but they do not have to be. You can go with a 64k size or even larger. The usual reason for larger pages is to avoid TLB misses. Again, I would make it a kernel configuration or run-time parameter.
Search for KERNEL_STACK_SIZE in linux kernel source code and you will find that it is very much architecture dependent - PAGE_SIZE, or 2*PAGE_SIZE etc (below is just some results - many intermediate output are deleted).
./arch/cris/include/asm/processor.h:
#define KERNEL_STACK_SIZE PAGE_SIZE
./arch/ia64/include/asm/ptrace.h:
# define KERNEL_STACK_SIZE_ORDER 3
# define KERNEL_STACK_SIZE_ORDER 2
# define KERNEL_STACK_SIZE_ORDER 1
# define KERNEL_STACK_SIZE_ORDER 0
#define IA64_STK_OFFSET ((1 << KERNEL_STACK_SIZE_ORDER)*PAGE_SIZE)
#define KERNEL_STACK_SIZE IA64_STK_OFFSET
./arch/ia64/include/asm/mca.h:
u64 mca_stack[KERNEL_STACK_SIZE/8];
u64 init_stack[KERNEL_STACK_SIZE/8];
./arch/ia64/include/asm/thread_info.h:
#define THREAD_SIZE KERNEL_STACK_SIZE
./arch/ia64/include/asm/mca_asm.h:
#define MCA_PT_REGS_OFFSET ALIGN16(KERNEL_STACK_SIZE-IA64_PT_REGS_SIZE)
./arch/parisc/include/asm/processor.h:
#define KERNEL_STACK_SIZE (4*PAGE_SIZE)
./arch/xtensa/include/asm/ptrace.h:
#define KERNEL_STACK_SIZE (2 * PAGE_SIZE)
./arch/microblaze/include/asm/processor.h:
# define KERNEL_STACK_SIZE 0x2000
I'll throw my two cents in to get the ball rolling:
I'm not sure what a "typical" stack size would be. I would guess maybe 8 KB per thread, and if a thread exceeds this amount, just throw an exception. However, according to this, Windows has a default reserved stack size of 1MB per thread, but it isn't committed all at once (pages are committed as they are needed). Additionally, you can request a different stack size for a given EXE at compile-time with a compiler directive. Not sure what Linux does, but I've seen references to 4 KB stacks (although I think this can be changed when you compile the kernel and I'm not sure what the default stack size is...)
This ties in with the first point. You probably want a fixed limit on how much stack each thread can get. Thus, you probably don't want to automatically allocate more stack space every time a thread exceeds its current stack space, because a buggy program that gets stuck in an infinite recursion is going to eat up all available memory.
If you are using virtual memory, you do want to make the stack growable. Forcing static allocation of stack sized, like is common in user-level threading like Qthreads and Windows Fibers is a mess. Hard to use, easy to crash. All modern OSes do grow the stack dynamically, I think usually by having a write-protected guard page or two below the current stack pointer. Writes there then tell the OS that the stack has stepped below its allocated space, and you allocate a new guard page below that and make the page that got hit writable. As long as no single function allocates more than a page of data, this works fine. Or you can use two or four guard pages to allow larger stack frames.
If you want a way to control stack size and your goal is a really controlled and efficient environment, but do not care about programming in the same style as Linux etc., go for a single-shot execution model where a task is started each time a relevant event is detected, runs to completion, and then stores any persistent data in its task data structure. In this way, all threads can share a single stack. Used in many slim real-time operating systems for automotive control and similar.
Why not make the stack size a configurable item, either stored with the program or specified when a process creates another process?
There are any number of ways you can make this configurable.
There's a guideline that states "0, 1 or n", meaning you should allow zero, one or any number (limited by other constraints such as memory) of an object - this applies to sizes of objects as well.