June 2016 Archives
I hinted at the end of the last post that the 6.7 release might increase performance a little. Well, whilst the bulk of the changes in 6.7 are purely code cleaning and the removal of legacy support there is a fairly major functional change as well.
In most situations references or pointers to I/O buffers have been replaced with smart pointers. This change may cause some issues during an upgrade as you need to change some function signatures from
IBuffer refs to
CSmartBuffers. The advantage is that in many servers there will no longer be any need for buffer reference counting during normal I/O operations.
The Server Framework relies on reference counting to keep the objects that are used during the asynchronous operations alive until those operations complete. So we increment a counter on the socket object and also on the buffer object when we initiate an operation and then decrement the counters when the operation completes. I'm sure there are other ways to manage the object lifetime but this has worked well for us.The problem is that these increments, although they look like cheap operations, can be quite expensive, especially on NUMA hardware.
Whilst there's not much we can do about the reference count on the socket object, the buffer doesn't really need to be reference counted most of the time. Or, more's the point. The initial reference can (and should) be passed along rather than each stage taking and releasing its own reference. With a buffer you generally only want to be accessing it from one thread at a time and so you allocate it and then issue an operation and pass the reference you have off to the operation. When the operation completes the code captures the reference and takes ownership of it and then the buffer can be processed. If you're lucky you can then use the same buffer for a response to the operation and pass it back to the framework again.
This requires a few changes to your code but it's fairly straight forward. Your
OnReadCompleted() handler will give you a
CSmartBuffer and if you want to retain ownership of it after the handler returns then you simply need to detach the buffer from the
CSmartBuffer you were given.
This is only "potentially faster" as it really depends on the structure of your server and how you deal with our I/O buffers but the framework is no longer standing in the way of this kind of optimisation, and we've removed a couple of reference adjustments in the normal operation flow.
We've only just shipped Release 6.6.5 of The Server Framework but we already have another release that's just about to ship. This isn't because some horrible bug has slipped through our testing, it's because we've been planning to produce a 'clean up' release for some time. 6.7 is that release.
Lets be straight here, 6.7 is a release for us more than for you. The aim is to simplify our build/test and release process, remove dead code whilst introducing no new bugs and removing no functionality that you rely on.
So what does 6.7 achieve. Well, for a start we drop support for Visual Studio 2005 and 2008 and also for Windows XP. Removing support for these legacy compilers and operating systems means that we can remove all the code that was required just to support them. This massively simplifies our code base without removing anything that the code actually relies on to run on modern operating systems.
Windows Vista introduced massively important changes to asynchronous I/O and we have supported these changes for a long time (over 8 years!). The code required to jump through hoops to make code running on Windows XP behave was complex. For example, Windows XP would cancel outstanding I/O requests if the thread that issued them exited before the I/O request completed. We had a marshalling system in place to ensure that I/O operations were only ever executed on threads that we controlled so that you'd never be faced with unexpectedly cancelled operations. All of that can go now.
Removing XP also means we no longer need to maintain an XP machine in our build farm. It's one less configuration that needs to be built and tested before a release.
Dropping support for VS2005 and 2008 removes 4 complete sets of builds (x86 and x64 for each compiler) plus all of the conditional code that was required to support the older compilers. At last we can start moving towards a slightly more modern C++, perhaps.
Some old code has been removed; there's no need, on modern operating systems, to share locks. This worked really well back in the day, but, well, we were running on Windows NT at the time and resources were much more limited than they are now. All of the "Shared Critical Section" code is now gone. This has knock on effects into the Socket Tools library where all of the shared lock socket code has been removed. Nobody should be using that in 2016 anyway! You can no longer set a critical section's spin count in the socket allocator, it never really worked anyway as the lock was used for too many different things.
Some experimental code has also been removed; The TLS and Low Contention buffer allocators are gone. The horrible "dispatch to all threads" cludge has been removed from the I/O pools (it was only there to support pre-Vista
CancelIO() calls which are no longer needed now that we have
The original callback timer queue that was based on
GetTickCount() and which spawned Len's "Practical testing" series of blog posts (back in 2004!) has gone. There's no need for the complexity when all supported operating systems have
Finally we've slimmed down our set of example servers. Removing servers which didn't add much value or which duplicated other examples. Again, this speeds our release process by speeding up the build and test stage as there are fewer servers to build and fewer tests to run.
So, what's in it for you? Well, a faster build/test/release cycle so new functionality and bug fixes can be released quicker and potentially faster code in some circumstances. There's no great rush to upgrade if you don't want to, but we'll be focusing on the 6.7 code base going forwards.