> > However, the non-header-only work won't do anything in a "clean build" > scenario.
I don't think this is true. If you look at how many independent .o files we build that scan those headers each time it should be clear that reducing the complexity of the header file reduces the compile time. A good example of heave .o files are the mesos tests that scan close to all of stout / libprocess for each test file. — *Joris Van Remoortere* Mesosphere On Tue, Feb 14, 2017 at 4:49 PM, Jeff Coffler < jeff.coff...@microsoft.com.invalid> wrote: > Hi Neil, > > This was discussed in the CXX Mesos Slack channel yesterday. > > Basically, the two are separate and independent. Regardless of stout work, > I anticipate that PCH work will dramatically speed up the Windows build > (and Linux too, although I have less experience in that area). I'm going to > run some benchmarks on a subset of the code to give a good "before/after" > idea of the speedup and report to the list. > > If stout non-header-only library work is done, this will do a fair amount > to speed up incremental builds (i.e. you just update implementation of a > stout method, and only the related C file is rebuilt). However, the > non-header-only work won't do anything in a "clean build" scenario. And, if > course, if you change the interface of a stout method, all bets are off and > you get to rebuild virtually the world. > > PCH, on the other hand, will speed up all compiles across the board (using > stout and not using stout). Now, that said, if a stout change is made > (assuming still header-only), you will still rebuild everything, but the > builds will go much faster. That *may* be fast enough to take the sting out > of significant stout changes, but changing stout will still help the > incremental build cases regardless. > > Hope that clarifies, > > /Jeff > > -----Original Message----- > From: Neil Conway [mailto:neil.con...@gmail.com] > Sent: Tuesday, February 14, 2017 11:45 AM > To: dev <dev@mesos.apache.org> > Subject: Re: Proposal for Mesos Build Improvements > > I'm curious to hear more about how using PCH compares with making stout a > non-header-only library. Is PCH easier to implement, or is it expected to > offer a more dramatic improvement in compile times? Would making both > changes eventually make sense? > > Neil > > On Tue, Feb 14, 2017 at 11:28 AM, Jeff Coffler <jeff.coff...@microsoft.com > .invalid> wrote: > > Proposal For Build Improvements > > > > The Mesos build process is in dire need of some build infrastructure > improvements. These improvements will improve speed and ease of work in > particular components, and dramatically improve overall build time, > especially in the Windows environment, but likely in the Linux environment > as well. > > > > > > Background: > > > > It is currently recommended to use the ccache project with the Mesos > build process. This makes the Linux build process more tolerable in terms > of speed, but unfortunately such software is not available on Windows. > Ultimately, though, the caching software is covering up two fundamental > flaws in the overall build process: > > > > 1. Lack of use of libraries > > 2. Lack of precompiled headers > > > > By not allowing use of libraries, the overall build process is often > much longer, particularly when a lot of work is being done in a particular > component. If work is being done in a particular component, only that > library need be rebuilt (and then the overall image relinked). Currently, > since there is no such modularization, all source files must be considered > at build time. Interestingly enough, there is such modularization in the > source code layout; that modularization just isn't utilized at the compiler > level. > > > > Precompiled headers exist on both Windows and Linux. For Linux, you can > refer to https://na01.safelinks.protection.outlook.com/?url= > https%3A%2F%2Fgcc.gnu.org%2Fonlinedocs%2Fgcc%2FPrecompiled-Headers.html& > data=02%7C01%7CJeff.Coffler%40microsoft.com%7Cf0dfa7d79e6e43d31fa008d45512 > 0381%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0% > 7C636226983234972044&sdata=ljS8BJ9ZSI7Wqvk5%2Bv1oPH5c6tHZGg7FPb08nUN8JUc% > 3D&reserved=0. Straight from the GNU CC documentation: "The time the > compiler takes to process these header files over and over again can > account for nearly all of the time required to build the project." > > > > In my prior use of precompiled headers, each C or C++ file generally > took about 4 seconds to compile. After switching to precompiled headers, > the precompiled header creation took about 4 seconds, but each C/C++ file > now took about 200 milliseconds to compile. The overall build speed was > thus dramatically reduced. > > > > > > Scope of Changes: > > > > These changes are only being proposed for the CMake system. Going > forward, the CMake system is the easiest way to maintain some level of > portability between the Linux and Windows platforms. > > > > > > Details for Modularization: > > > > For the modularization, the intent is to simply make each source > directory of files, if functionally separate, to be compiled into an > archive (.a) file. These archive files will then be linked together to form > the actual executables. These changes will primarily be in the CMake > system, and should have limited effect on any actual source code. > > > > At a later date, if it makes sense, we can look at building shared > library (.so) files. However, this only makes the most sense if the code is > truly shared between different executable files. If that's not the case, > then it likely makes sense just to stick with .a files. Regardless, > generation of .so files is out of scope for this change. > > > > > > Details for Precompiled Header Changes: > > > > Precompiled headers will make use of stout (a very large header-only > library) essentially "free" from a compile-time overhead point of view. > Basically, precompiled headers will take a list of header files (including > very long header files, like "windows.h"), and generate the compiler memory > structures for their representation. > > > > During precompiled header generation, these memory structures are > flushed to disk. Then, when components are built, the memory structures are > reloaded from disk, which is dramatically faster than actually parsing the > tens of thousands of lines of header files and building the memory > structures. > > > > For precompiled headers to be useful, a relatively "consistent" set of > headers must be included by all of the C/C++ files. So, for example, > consider the following C file: > > > > #if defined(windows) > > #include <windows.h> > > #endif > > > > #include <header-a> > > #include <header-b> > > #include <header-c> > > > > < - Remainder of module - > > > > > To make a precompiled header for this module, all of the #include files > would be included in a new file, mesos_common.h. The C file would then be > changed as follows: > > > > #include "mesos_common.h" > > > > < - Remainder of module - > > > > > Structurally, the code is identical, and need not be built with > precompiled headers. However, use of precompiled headers will make file > compilation dramatically faster. > > > > Note that other include files can be included after the precompiled > header if appropriate. For example, the following is valid: > > > > #include "mesos_common.h" > > #inclue <header-d> > > > > < - Remainder of module - > > > > > For efficiency purposes, if a header file is included by 50% or more of > the source files, it should be included in the precompiled header. If a > header is included in fewer than 50% of the source files, then it can be > separately included (and thus would not benefit from precompiled headers). > Note that this is a guideline; even if a header is used by less than 50% of > source files, if it's very large, we still may decide to throw it in the > precompiled header. > > > > Note that, for use of precompiled headers, there will be a great deal > > of code churn (almost exclusively in the #include list of source > > files). This will mean that there will be a lot of code merges, but > > ultimately no "code logic" changes. If merges are not done in a timely > > fashion, this can easily result in needless hand merging of changes. > > Due to these issues, we will need a dedicated sheppard that will > > integrate the patches quickly. This kind of work is easily invalidated > > when the include list is changed by another developer, necessitating > > us to redo the patch. [Note that Joseph has stepped up to the plate > > for this, thanks Joseph!] > > > > > > This is the end of my proposal, feedback would be appreciated. >
