Using std::async for easy parallel computations
C++0x, the next major revision of C++, includes a number of new language and library facilities that I am greatly looking forward to, including a standard thread interface. Initially the agenda for C++0x had included facilities built on threads, such as a thread pool, but as part of the so-called 'Kona compromise' (named after the location of the committee meeting where the compromise was made) all but the most basic facilities were deferred for a later revision.
However there were many requests for a simple facility for creating
an asynchronous function call, and a function for this, named
std::async, was voted in at the last meeting.
std::async is a rather blunt tool; it spawns a new thread
(though wording is included which would allow an implementation to
spawn threads in a fixed-size thread pool to eliminate thread creation
overhead and reduce hardware oversubscription) and returns a "future"
representing the return value of the function. A future is a
placeholder for a value which can be passed around the program, and if
and when the value is actually needed, it can be retrieved from the
future; the get operation may block if the value has not yet
been computed. In C++0x the future/promise system is primarily
intended for use with threads, but there doesn't seem to be any reason
a system for distributed RPC (ala E's Pluribus protocol) could not
provide an interface using the same classes.
An operation which felt like easy low-hanging fruit for parallel
invocation is RSA's decrypt/sign operation. Mathematically, when one
signs a message using RSA, the message representation (usually a hash
function output plus some specialized padding) is converted to an
integer, and then raised to the power of d, the RSA private
key, modulo another number. Both of these numbers are relatively
large, typically 300 to 600 digits long. A well known trick, which
takes advantage of the underlying structure of the numbers, allows one
to instead compute two modular exponentiations, both using numbers
about half the size of d, and combine them using the Chinese
Remainder Theorem (thus this optimization is often called
RSA-CRT). The two computations are both still quite intensive, and
since they are independent it seemed reasonable to try computing them
in parallel. Running one of the two exponentiations in a different
thread showed an immediate doubling in speed for RSA signing on a
multicore! Other mathematically intensive algorithms that offer some
amount of parallel computation, including DSA and ElGamal, also showed
nice improvements.
As std::async is not included in GCC 4.5, I wrote a simple
clone of it. This version does not offer thread pooling or the option
of telling the runtime to run the function on the same thread; it is
mostly a 'proof of concept' version I'm using until GCC includes the
real deal in libstdc++. Here is the code:
#include <future>
#include <thread>
template<typename F>
auto std_async(F f) -> std::unique_future<decltype(f())>
{
typedef decltype(f()) result_type;
std::packaged_task<result_type ()> task(std::move(f));
std::unique_future<result_type> future = task.get_future();
std::thread thread(std::move(task));
thread.detach();
return future;
}
The highly curious auto return type of std_async
uses C++0x's new function declaration syntax; ordinarily there is
no reason to use it but here we want to specify that the function
returns a unique_future paramaterized by whatever it is
that f returns. Since f can't be referred to until
it has been mentioned as the name of an argument, the return value
has to come after the parameter list.
Unlike the version of std::async that was finally voted
in, std_async assumes its argument takes no arguments (one of
the original proposals for std::async used a similar
interface). This would be highly inconvenient except for the
assistance of C++0x's lambdas, which allow us to pack everything
together. For instance here is the code for RSA signing, which
packages up one half of the computation in a 0-ary lambda
function:
auto future_j1 = std_async([&]() { return powermod_d1_p(i); });
BigInt j2 = powermod_d2_q(i);
BigInt j1 = future_j1.get();
// Now combine j1 and j2 using CRT
Using C++0x's std::bind instead of a lambda here should
work as well, but I ran into problem with that in the 4.5 snapshot I'm
using; the current implementation follows the TR1 style of requiring
result_type typedefs which will not be necessary in C++0x
thanks to decltype. Since the actual std::async can
take an arbitrary number of arguments, the declaration of
future_j1 will eventually change to simply:
auto future_j1 = std::async(powermod_d1_p, i);
The implementation of std_async may strike you as
excessively C++0x-ish, for instance by using decltype instead
of TR1's result_of metaprogramming function. Part of this is
due to current limitations of GCC and/or libstdc++; the version of
result_of in 4.5's libstdc++ does not understand lambda
functions (C++0x's result_of is guaranteed to get this right,
because it itself uses decltype, but apparently libstdc++
hasn't changed to use this yet).
Overall I'm pretty happy with C++0x as an evolution of C++98 for systems programming tasks. Though I am certainly interested to see how Thompson and Pike's Go works out; now that BitC is more or less dead after the departure of its designers to Microsoft, Go seems to be the only game in town in terms of new systems programming languages that might provide a compelling alternative to C++.
Posted in programming at 2009/11/24 10:09; 1 comment
Converting Line Endings in InnoSetup
I recently packaged botan for Windows using InnoSetup, an open source installation creator. Overall I was pretty pleased with it - it seems to do everything I need it to do without much of a hassle, and I'll probably use it in the future if I need to package other programs or tools for Windows.
After I got the basic package working, a nit I wanted to deal with was converting the line endings of all the header files and plain-text documentation (readme, license file, etc) to use Windows line endings. While many Windows programs, including Wordpad and Visual Studio, can deal with files with Unix line endings, not all do, and it seemed like it would be a nice touch if the files were not completely unreadable if opened in Notepad.
There is no built in support for this, but InnoSetup includes a scripting facility (using Pascal!), including hooks that can be called at various points in the installation process, including immediately after a file is installed, which handles this sort of problem perfectly. So all that was required was to learn enough Pascal to write the function. I've included it below to help anyone who might be searching for a similar facility, since my own searches looking for an example of doing this were fruitless:
[Code]
const
LF = #10;
CR = #13;
CRLF = CR + LF;
procedure ConvertLineEndings();
var
FilePath : String;
FileContents : String;
begin
FilePath := ExpandConstant(CurrentFileName)
LoadStringFromFile(FilePath, FileContents);
StringChangeEx(FileContents, LF, CRLF, False);
SaveStringToFile(FilePath, FileContents, False);
end;
Adding the hook with AfterInstall: ConvertLineEndings
caused this function to run on each of my text and include files.
Posted in programming at 2009/11/23 18:51; 0 comments
SSE2 Serpent on Atom N270: twice as fast as AES-128
On the Intel Atom N270 processor, OpenSSL 0.9.8g's implementation
of AES-128 runs at 25 MiB per second (CBC mode, using openssl
speed). In contrast, the Serpent implementation using SSE2
I described last
month runs at over 60 MiB per second in ECB mode (2.4x faster) and
48 MiB per second in CTR mode (1.9x faster).
Posted in programming at 2009/10/21 02:11; 0 comments
Programming trivia: 4x4 integer matrix transpose in SSE2
The Intel SSE2 intrinsics has a macro _MM_TRANSPOSE4_PS
which performs a matrix transposition on a 4x4 array represented by
elements in 4 SSE registers. However, it doesn't work with integer
registers because Intel intrinsics make a distinction between integer
and floating point SSE registers. Theoretically one could cast and use
the floating point operations, but it seems quite plausible that this
will not round trip properly; for instance if one of your integer
values happens to have the same value as a 32-bit IEEE denormal.
However it is easy to do with the punpckldq, punpckhdq, punpcklqdq, and punpckhqdq instructions; code and diagrams ahoy.
Posted in programming at 2009/10/08 17:50; 0 comments
After the initial set of attacks on MD5 and SHA-1, NIST organized a series of conferences on hash function design. I was lucky enough to be able to attend the first one, and had a great time. This was the place where the suggestion of a competition in the style of the AES process to replace SHA-1 and SHA-2 was first proposed (to wide approval). This has resulted in over 60 submissions to the SHA-3 contest, of which 14 have been brought into the second round.
Of the second round contenders, I think Skein is the best choice for becoming SHA-3, and want to explain why I think so.
Posted in security at 2009/10/08 15:19; 0 comments
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