Static 链接与动态链接

Question

Are there any compelling performance reasons to choose static linking over dynamic linking or vice versa in certain situations? I've heard or read the following, but I don't know enough on the subject to vouch for its veracity.

1) The difference in runtime performance between static linking and dynamic linking is usually negligible.

2) (1) is not true if using a profiling compiler that uses profile data to optimize program hotpaths because with static linking, the compiler can optimize both your code and the library code. With dynamic linking only your code can be optimized. If most of the time is spent running library code, this can make a big difference. Otherwise, (1) still applies.

Answer 1

• Dynamic linking can reduce total resource consumption (if more than one process shares the same library (including the version in "the same", of course)). I believe this is the argument that drives it its presence in most environments. Here "resources" includes disk space, RAM, and cache space. Of course, if your dynamic linker is insufficiently flexible there is a risk of DLL hell .
• Dynamic linking means that bug fixes and upgrades to libraries propagate to improve your product without requiring you to ship anything.
• Plugins always call for dynamic linking.
• Static linking, means that you can know the code will run in very limited environments (early in the boot process, or in rescue mode).
• Static linking can make binaries easier to distribute to diverse user environments (at the cost of sending a larger and more resource hungry program).
• Static linking may allow slightly faster startup times, but this depends to some degree on both the size and complexity of your program and on the details of the OS's loading strategy.

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Some edits to include the very relevant suggestions in the comments and in other answers. I'd like to note that the way you break on this depends a lot on what environment you plan to run in. Minimal embedded systems may not have enough resources to support dynamic linking. Slightly larger small systems may well support dynamic linking, because their memory is small enough to make the RAM savings from dynamic linking very attractive. Full blown consumer PCs have, as Mark notes, enormous resources, and you can probably let the convenience issues drive your thinking on this matter.

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To address the performance and efficiency issues: it depends .

Classically, dynamic libraries require a some kind of glue layer which often means double dispatch or an extra layer of indirection in function addressing and can cost a little speed (but is function calling time actually a big part of your running time???).

However, if you are running multiple processes which all call the same library a lot, you can end up saving cache lines (and thus winning on running performance) when using dynamic linking relative to using static linking. (Unless modern OS's are smart enough to notice identical segments in statically linked binaries. Seems hard, anyone know?)

Another issue: loading time. You pay loading costs at some point. When you pay this cost depends on how the OS works as well as what linking you use. Maybe you'd rather put off paying it until you know you need it.

Note that static-vs-dynamic linking is traditionally not an optimization issue, because they both involve separate compilation down to object files. However, this is not required: a compiler can in principle, "compile" "static libraries" to a digested AST form initially, and "link" them by adding those ASTs to the ones generated for the main code, thus empowering global optimization. None of the systems I use do this, so I can't comment on how well it works.

The way to answer performance questions is always by testing (and use a test environment as much like the deployment environment as possible).

Answer 2

1) is based on the fact that calling a DLL function is always using an extra indirect jump. Today, this is usually negligible. Inside the DLL there is some more overhead on i386 CPU's, because they can't generate position independent code. On amd64, jumps can be relative to the program counter, so this is a huge improvement.

2) This is correct. With optimizations guided by profiling you can usually win about 10-15 percent performance. Now that CPU speed has reached its limits it might be worth doing it.

I would add: (3) the linker can arrange functions in a more cache efficient grouping, so that expensive cache level misses are minimised. It also might especially effect the startup time of applications (based on results i have seen with the Sun C++ compiler)

And don't forget that with DLLs no dead code elimination can be performed. Depending on the language, the DLL code might not be optimal either. Virtual functions are always virtual because the compiler doesn't know whether a client is overwriting it.

For these reasons, in case there is no real need for DLLs, then just use static compilation.

EDIT (to answer the comment, by user underscore)

Here is a good resource about the position independent code problem http://eli.thegreenplace.net/2011/11/03/position-independent-code-pic-in-shared-libraries/

As explained x86 does not have them AFAIK for anything else then 15 bit jump ranges and not for unconditional jumps and calls. That's why functions (from generators) having more then 32K have always been a problem and needed embedded trampolines.

But on popular x86 OS like Linux you do not need to care if the .so/DLL file is not generated with the gcc switch -fpic (which enforces the use of the indirect jump tables). Because if you don't, the code is just fixed like a normal linker would relocate it. But while doing this it makes the code segment non shareable and it would need a full mapping of the code from disk into memory and touching it all before it can be used (emptying most of the caches, hitting TLBs) etc. There was a time when this was considered slow.

So you would not have any benefit anymore.

I do not recall what OS (Solaris or FreeBSD) gave me problems with my Unix build system because I just wasn't doing this and wondered why it crashed until I applied -fPIC to gcc .

Answer 3

动态链接是满足某些许可要求（例如LGPL ）的唯一实用方法。

Answer 4

I agree with the points dnmckee mentions, plus:

• Statically linked applications might be easier to deploy, since there are fewer or no additional file dependencies (.dll / .so) that might cause problems when they're missing or installed in the wrong place.

Answer 5

One reason to do a statically linked build is to verify that you have full closure for the executable, ie that all symbol references are resolved correctly.

As a part of a large system that was being built and tested using continuous integration, the nightly regression tests were run using a statically linked version of the executables. Occasionally, we would see that a symbol would not resolve and the static link would fail even though the dynamically linked executable would link successfully.

This was usually occurring when symbols that were deep seated within the shared libs had a misspelt name and so would not statically link. The dynamic linker does not completely resolve all symbols, irrespective of using depth-first or breadth-first evaluation, so you can finish up with a dynamically linked executable that does not have full closure.

Answer 6

1/ I've been on projects where dynamic linking vs static linking was benchmarked and the difference wasn't determined small enough to switch to dynamic linking (I wasn't part of the test, I just know the conclusion)

2/ Dynamic linking is often associated with PIC (Position Independent Code, code which doesn't need to be modified depending on the address at which it is loaded). Depending on the architecture PIC may bring another slowdown but is needed in order to get benefit of sharing a dynamically linked library between two executable (and even two process of the same executable if the OS use randomization of load address as a security measure). I'm not sure that all OS allow to separate the two concepts, but Solaris and Linux do and ISTR that HP-UX does as well.

3/ I've been on other projects which used dynamic linking for the "easy patch" feature. But this "easy patch" makes the distribution of small fix a little easier and of complicated one a versioning nightmare. We often ended up by having to push everything plus having to track problems at customer site because the wrong version was token.

My conclusion is that I'd used static linking excepted:

• for things like plugins which depend on dynamic linking

• when sharing is important (big libraries used by multiple processes at the same time like C/C++ runtime, GUI libraries, ... which often are managed independently and for which the ABI is strictly defined)

If one want to use the "easy patch", I'd argue that the libraries have to be managed like the big libraries above: they must be nearly independent with a defined ABI that must not to be changed by fixes.

Answer 7

这将详细讨论 linux 上的共享库和性能影响。

Answer 8

It is pretty simple, really. When you make a change in your source code, do you want to wait 10 minutes for it to build or 20 seconds? Twenty seconds is all I can put up with. Beyond that, I either get out the sword or start thinking about how I can use separate compilation and linking to bring it back into the comfort zone.

Answer 9

Best example for dynamic linking is, when the library is dependent on the used hardware. In ancient times the C math library was decided to be dynamic, so that each platform can use all processor capabilities to optimize it.

An even better example might be OpenGL. OpenGl is an API that is implemented differently by AMD and NVidia. And you are not able to use an NVidia implementation on an AMD card, because the hardware is different. You cannot link OpenGL statically into your program, because of that. Dynamic linking is used here to let the API be optimized for all platforms.

Answer 10

On Unix-like systems, dynamic linking can make life difficult for 'root' to use an application with the shared libraries installed in out-of-the-way locations. This is because the dynamic linker generally won't pay attention to LD_LIBRARY_PATH or its equivalent for processes with root privileges. Sometimes, then, static linking saves the day.

Alternatively, the installation process has to locate the libraries, but that can make it difficult for multiple versions of the software to coexist on the machine.

Answer 11

Static linking is a process in compile time when a linked content is copied into the primary binary and becomes a single binary.

Cons:

• compile time is longer
• output binary is bigger

Dynamic linking is a process in runtime when a linked content is loaded. This technic allows to:

• upgrade linked binary without recompiling a primary one that increase an ABI stability ^[About]
• has a single shared copy

Cons:

• start time is slower(linked content should be copied)
• linker errors are thrown in runtime

[iOS Static vs Dynamic framework]

Answer 12

Dynamic linking requires extra time for the OS to find the dynamic library and load it. With static linking, everything is together and it is a one-shot load into memory.

Also, see DLL Hell . This is the scenario where the DLL that the OS loads is not the one that came with your application, or the version that your application expects.

Answer 13

Another issue not yet discussed is fixing bugs in the library.

With static linking, you not only have to rebuild the library, but will have to relink and redestribute the executable. If the library is just used in one executable, this may not be an issue. But the more executables that need to be relinked and redistributed, the bigger the pain is.

With dynamic linking, you just rebuild and redistribute the dynamic library and you are done.

Answer 14

Static linking includes the files that the program needs in a single executable file.

Dynamic linking is what you would consider the usual, it makes an executable that still requires DLLs and such to be in the same directory (or the DLLs could be in the system folder).

(DLL = dynamic link library)

Dynamically linked executables are compiled faster and aren't as resource-heavy.

Answer 15

static linking gives you only a single exe, inorder to make a change you need to recompile your whole program. Whereas in dynamic linking you need to make change only to the dll and when you run your exe, the changes would be picked up at runtime.Its easier to provide updates and bug fixes by dynamic linking (eg: windows).

Answer 16

There are a vast and increasing number of systems where an extreme level of static linking can have an enormous positive impact on applications and system performance.

I refer to what are often called "embedded systems", many of which are now increasingly using general-purpose operating systems, and these systems are used for everything imaginable.

An extremely common example are devices using GNU/Linux systems using Busybox . I've taken this to the extreme with NetBSD by building a bootable i386 (32-bit) system image that includes both a kernel and its root filesystem, the latter which contains a single static-linked (by crunchgen ) binary with hard-links to all programs that itself contains all (well at last count 274) of the standard full-feature system programs (most except the toolchain), and it is less than 20 mega bytes in size (and probably runs very comfortably in a system with only 64MB of memory (even with the root filesystem uncompressed and entirely in RAM), though I've been unable to find one so small to test it on).

It has been mentioned in earlier posts that the start-up time of a static-linked binaries is faster (and it can be a lot faster), but that is only part of the picture, especially when all object code is linked into the same file, and even more especially when the operating system supports demand paging of code direct from the executable file. In this ideal scenario the startup time of programs is literally negligible since almost all pages of code will already be in memory and be in use by the shell (and and init any other background processes that might be running), even if the requested program has not ever been run since boot since perhaps only one page of memory need be loaded to fulfill the runtime requirements of the program.

However that's still not the whole story. I also usually build and use the NetBSD operating system installs for my full development systems by static-linking all binaries. Even though this takes a tremendous amount more disk space (~6.6GB total for x86_64 with everything, including toolchain and X11 static-linked) (especially if one keeps full debug symbol tables available for all programs another ~2.5GB), the result still runs faster overall, and for some tasks even uses less memory than a typical dynamic-linked system that purports to share library code pages. Disk is cheap (even fast disk), and memory to cache frequently used disk files is also relatively cheap, but CPU cycles really are not, and paying the ld.so startup cost for every process that starts every time it starts will take hours and hours of CPU cycles away from tasks which require starting many processes, especially when the same programs are used over and over, such as compilers on a development system. Static-linked toolchain programs can reduce whole-OS multi-architecture build times for my systems by hours . I have yet to build the toolchain into my single crunchgen 'ed binary, but I suspect when I do there will be more hours of build time saved because of the win for the CPU cache.

Answer 17

Another consideration is the number of object files (translation units) that you actually consume in a library vs the total number available. If a library is built from many object files, but you only use symbols from a few of them, this might be an argument for favoring static linking, since you only link the objects that you use when you static link (typically) and don't normally carry the unused symbols. If you go with a shared lib, that lib contains all translation units and could be much larger than what you want or need.