Kernel events without kevents [LWN.net]

Kernel events without kevents

Posted Mar 17, 2007 1:50 UTC (Sat) by mikov (subscriber, #33179)
In reply to: Kernel events without kevents by pphaneuf
Parent article: Kernel events without kevents

Well, I have to respectfully disagree (let's be extra careful not to get into a silly Win32 vs POSIX or whatever flame :-)

WaitForMultipleObjects() is not ideal, of course, and has limitations (personally I have never struggled with them after quite a few years of system programming for Win32), but they also come with their known workarounds. However it is nothing if not consistent and easy to use. It fits perfectly with the rest of the model - files, async. (overlapped), operations, threads, etc. There is never question of what is the correct way to implement something and more importantly there is no need for ridiculous hacks like using pipes from signals, etc.

Usually people who haven't seriously used the Base (*) Win32 API tend to underestimate it (I am not implying that you are one of them), but some parts of it are quite good, IMHO. As I said, the most important quality in my mind is that it is really well thought out and everything fits together. There are no corner cases which do not work. By comparison Linux or POSIX is has a few non-obvious problems and solutions - it shows that it has grown evolutionary.

(*) By the "base" Win32 API I mean pretty much only the IO, synchronzation and threads. Those parts have remained consistent and stable for as long as I remember. The rest of the Win32 API (the GUI, COM, etc) is of course a complete nightmare.

About the Ex functions. AFAIK, they were not added later, they simply provide different functionality. For example ReadFileEx() is not a newer more powerful version of ReadFile() - it just operates with a different model. More importantly, you can't emulate ReadFile() using ReadFileEx()! (Well, I could be wrong about when they were added ReadFileEx, since I haven't exactly tracked the API changes - it uses a fundamental concept though)

I agree with you that poll() and epoll() are more powerfull/convenient than WaitForMultipleObjects(). However they stand alone - you can't use them (yet) to wait for a signal, for a completion of a child process, for a semaphore (or has changed already?), etc. That is the problem !

BTW, I also tend to disagree about the utility of the WNDPROC in a hidden window for purposes like you describe. The whole UI paradigm in Win32 is a remnant from Win16, and is more or less incompatible with the rest of the API - thus the need for "hacks" like MsgWaitForMultipleObjects().

In my Windows code I've always tried at all cost to avoid using window messages for anything besides "pure" UI - in my experience they make the code very fragile (and obviously non-portable). You also have the problem that UI operations performance can limit your background tasks. I am not aware of any contemporary Win32 non-GUI API that relies on messages. (Win16 sockets used to, but that has been deprecated in Win32)

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Kernel events without kevents

Posted Mar 17, 2007 11:59 UTC (Sat) by pphaneuf (guest, #23480) [Link]

64 is a pretty low number of objects, I'd say, but it depends on the design. It seems more oriented to a "one thread per connection" design, where a single connection could be handling a few things (waiting for an answer from a database, but also watching the socket for disconnection, say). Also, the way it can easily lead to starvation seems like a major design problem with it. Careless ordering of the list of handles could lead to a stuck process! Odd, that.

I both agree and disagree with your reply. Win32 had advanced capabilities for a long time, such as completion ports (although I think they were severely crippled on the non-NT platforms, if I recall correctly). We still don't have many of those on Linux, and POSIX itself is so out of date, it's practically irrelevant. If you want to do something high-performance that works on multiple Unix platforms, you have to invent your own abstraction for the various high-performance APIs, because POSIX is just too pathetic. Sure, you can also make a POSIX version for complete portability, but you just know that this one isn't going to be the one for high-performance requirements.

You say

It's a kind strange, comparing the two APIs. I feel that the Win32 is a bit more integrated, yes, having had this stuff for a longer time (everything is an object with a handle, that can be given to WaitForMultipleObjects()), but somehow doesn't feel like that (reading from a socket with ReadFile() seems a bit odd).

The classic Unix API feels more tasteful, but seems to suffer from some rot. For example, newer additions, like threads, feels like they are crummy copies of other APIs, not fitting in well with the rest. For example, what you say about poll() not being able to wait on a number of things is really mostly linked to those new things (mostly related to threads) not having been made file descriptors in the first place! Remember that file descriptors are more or less the Unix equivalent of handles, despite having the name "file" in it (it's the "everything is a file" concept).

The signals are the exception (and thus, waiting for child processes), but that's by design. There are only two ways to affect a Unix process, synchronously (but not necessarily blocking) through a file descriptor, or asynchronously through a signal. And there are bridges to go from one to another (SIGIO to make file descriptor asynchronous, and pipes to turn signals into a synchronous event). The signal and pipe trick might sound hacky, but really, it's a matter of keeping the core simple, providing lightweight primitives that can be built upon to make the same effect.

It helps to remember Windows NT heritage from VMS, which had a number of distinct file types, that were opened and accessed completely differently, including a B-Tree file type. Think about it: on VMS, there was the equivalent of Berkeley DB in the kernel! Where on Unix, the philosophy is more that we'll give you the tools to write Berkeley DB, and you go from there. Hence my not being incredibly excited about signalfd(): it's nice, but not exceedingly so, since I could easily do without. In my Windows code I've always tried at all cost to avoid using window messages for anything besides "pure" UI - in my experience they make the code very fragile (and obviously non-portable). You also have the problem that UI operations performance can limit your background tasks. I am not aware of any contemporary Win32 non-GUI API that relies on messages. (Win16 sockets used to, but that has been deprecated in Win32)

You mention UI operations limiting your "background" tasks. This is always true, for a single thread, as really there isn't one that's foreground and the other that's background, they're all equals, competing for execution on a thread. If your UI is giving you grief, the answer isn't necessarily to stop using messages, but to have another thread, so that there's more execution contexts to do the work. Ideally, it would all be so uniform that UI code could run on any thread as well, so things would just get done as quickly as possible, no matter what it is, nothing having the edge over the other.

Kernel events without kevents

Posted Mar 17, 2007 18:35 UTC (Sat) by mikov (subscriber, #33179) [Link]

64 is a pretty low number of objects, I'd say, but it depends on the design. It seems more oriented to a "one thread per connection" design, where a single connection could be handling a few things (waiting for an answer from a database, but also watching the socket for disconnection, say). Also, the way it can easily lead to starvation seems like a major design problem with it. Careless ordering of the list of handles could lead to a stuck process! Odd, that.

Well, I've never had to use WaitForMultipleObjects() with more than a few handles. I think that for a single-threaded IO server design (which admittedly I haven't done), I'd use the ReadFileEx family of functions, which register a completion callback (APC) - this elliminates the 64 handle problem. Then I'd use WaitForMultipleObjectsEx() if I have to let the callbacks run _and_ wait for semaphores and stuff.

Addmitedly, since Win32 has always had well integrated threads, the need to shoehorn everything into a single thread has never been very strong.

It's a kind strange, comparing the two APIs. I feel that the Win32 is a bit more integrated, yes, having had this stuff for a longer time (everything is an object with a handle, that can be given to WaitForMultipleObjects()), but somehow doesn't feel like that (reading from a socket with ReadFile() seems a bit odd).

I guess it is force of habit. The other methods are also available - recv(),read(), etc. BTW, for some things the Win32 API can be a major PITA. For example serial communication - this is where you don't want overlapped operations and WaitForMultipleObjects - instead you really want select(). Alas, in Win32 select() works only on sockets ...

If your UI is giving you grief, the answer isn't necessarily to stop using messages, but to have another thread, so that there's more execution contexts to do the work. Ideally, it would all be so uniform that UI code could run on any thread as well, so things would just get done as quickly as possible, no matter what it is, nothing having the edge over the other.

But that's exactly it. In the main thread of a GUI application you need messages to drive the GUI. However if you create another thread (let's say for IO), there is no point at all in using Windows messages there. None of the APIs generate them, so you'd have to write code to send them yourself, make a message loop to handle them, etc. What's the point ? If you really needed some sort of of message queue in your design, there is zero reason to use the Windows GUI one - you are much better off coding something custom.