Readers-writer lock - Part 4: Improving TLightweightMREW

While the TLightweightMREW is quite handy, it is not perfect. There's a weird assymmetry in it. On all operating systems that Delphi can compile for, read locks are reentrant (recursive) while write locks are not. In other words, if a thread already owns a read lock, it can call BeginRead again and it will succeed. Write locks are different. If a thread already owns a write lock and calls BeginWrite again, it will either deadlock (on Windows) or raise an exception (on other supported platforms).

This is, however, relatively simple to fix. I have implemented a simple wrapper for the TLightweightMREW lock in TLightweightMREWEx. This new record uses internal TLightweightMREW to provide locking and adds some simple logic to implement write lock reentrancy. The implementation and accompanying test program rwReentrantWriter can be found at https://github.com/gabr42/examples/tree/master/Reader-writer%20lock.

Readers-writer lock - Part 3: Some numbers

In order to convince you that a readers-writer lock is not a stupid idea, I should finally show some numbers. In this article I'll present a minimalistic (but still real-life) example which allows us to compare different locking solutions.

All code from this article is available in project rwLock at  https://github.com/gabr42/examples/tree/master/Reader-writer%20lock

Readers-writer lock - Part 2: Implementation

In the previous installment I introduced the idea of a readers-writer lock. Today I'll look into readers-writer lock implementations (yes, multiple) that are available in the Delphi run-time library.

Readers-writer lock - Part 1: Why?

One of the pleasant surprises in Delphi 10.4.1 was the addition of a new readers-writer lock implementation TLightweightMREW. While it was probably not noticed by most of the users, I was quite happy to see it implemented. 

So now you are asking yourself - what is this readers-writer lock and why am I so happy to see it in Delphi? Well, I'm glad that you're asking! Let me explain ...

In multithreaded programming (as most of my horror stories start), we frequently run into a problem of resource sharing. Two threads want to modify a shared resource at the same time and that can cause many problems, from information being overwritten to corrupted data and program crashes.

To fix this, we add resource protection. Usually that is just a critical section (typically through a TCriticalSection wrapper), or Delphi's TMonitor. Sometimes, however, protecting resources with a simple critical section causes an unnecessary performance drop, and that's when a readers-writer lock (may) come into play.

OmniThreadLibrary 3.07.8

New OmniThreadLibrary is out! Get it while it’s hot!

Version 3.07.8 is mostly a bugfix release. It fixes few small bugs and enables support for Delphi 10.4.

You can get it now on git, download the ZIP archive, install it with Delphinus or with GetIt (in few days).

For more information, visit OmniThreadLibrary home page or write your question on the forum.

Top three Delphi 10.4 features

Delphi 10.4 has just been released (turn here for a great overview) and has some nice enhancements even for us, die-hard Windows developers. It is too early to give any deep analysis as I have just installed it and did not do any thorough testing, but I can already pick my top three new features. In no particular order, here they are:

Long live Delphi!

Something great has happened on this day, 25 years ago.

It was the sign of Aquarius. People were listening to Creep. And the AppBuilder was released.

You don’t know AppBuilder? Sure you do! It was developed and — thanks to Novell — released under codename Delphi.

Indeed, our beloved Delphi is 25 years old today! A quarter of a century!

Sales! Sales! Sales!

Parallel Programming with OmniThreadLibrary

e-book: 50% off

printed book: 25% off (use coupon BLACKFRIDAY25)

Delphi High Performance

e-book: 72% off

Hands-on Design Patterns with Delphi

e-book: 80% off

ITDevCon X

The ITDevCon X - the tenth edition! - has just ended and boy was it a blast! (As usual, I should say.)

This was my seventh ITDevCon conference and I always like to visit it. This year was no exception.

Thank you Daniele, Valentina, Fabrizio, and all bit Time gals and guys for organizing two days of great fun and great food!

P.S. My slides & code are already available online.

CompareValue for booleans

CompareValue function is incredibly practical when you are writing comparers (functions that determine how some data structure is ordered). System.Math and System.StrUtils define a bunch of functions that can be used to compare integers, doubles, strings … There’s, however, no CompareValue for booleans.

A CompareValue function compares two parameters, traditionally named left and right, and returns 0 if they are the same, –1 if left is smaller and 1 if right is smaller.

If we use the usual ordering of false < true, we can write the missing function as follows:

function CompareValue(left, right: boolean): integer; overload;
begin
  if left < right then
    Result := -1
  else if left > right then
    Result := 1
  else
    Result := 0;
end;

Your task for today – if you choose to accept it – is: Write this function without any if statements.

Visit “What’s new in Rio 10.3.2” in Ljubljana

On September 26^th I’ll talk about RAD Studio Rio in Ljubljana. We’ll discuss 10.3 a bit and 10.3.1/10.3.2 updates in more detail. We’ll also look into the future to see what 10.4 might bring.

This will also be a good opportunity to see my latest book, Design patterns with Delphi, or get your own copy signed.

Participation is free, but you should register here so we can prepare enough food for everyone.

When True is not

Pop quiz! How can the following program …

Writeln(True);
Magic;
Writeln(True);

… output this:?

TRUE
FALSE

Simple!

The case of a missing begin/end

Delphi never stops surprising me …
Did you know that this is a valid syntax?

case a of
  0: Writeln(0);
  else
    Writeln('else');
    Writeln(a);
end;

This code indeed compiles and works exactly as the following fragment.

case a of
  0: Writeln(0);
  else begin
    Writeln('else');
    Writeln(a);
  end;
end;

I personally would never drop begin/end inside a case/else statement, but at least someone must disagree. I found such example in a very (VERY!) old code (it was written for Delphi 2) and I was quite surprised that it compiles at all.

EDIT

Anton Alisov suggested formatting first example as:

case a of
  0: Writeln(0);
else
  Writeln('else');
  Writeln(a);
end;

I guess this makes more sense (but just an itsy bitsy teenie weenie bit more).

Delphi Day 2019 sessions

Slides and code for my Delphi Day 2019 sessions are now online.

Spring4D European Conference 2019 sessions

Slides and code for the Spring4D conference are now published on the conference page.

Slides and code for my two sessions – Design patterns with Spring4D and Interception and dynamic proxy – are also available on my presentations page.

FastMM4 large memory allocation–benchmarking VirtualAlloc

Earlier this week a long-time customer asked me why FastMM allocates large memory blocks from the top of the memory and if that option could safely be turned off. Their reasoning was that such allocations are much slower than normal ones. The question surprised me as I didn’t know of any such difference in speed so I did what I usually do–I wrote a benchmark application and measured it.

TL;DR Yes, allocating from the top is slower. No, the difference is not big and in most cases you’ll not even notice it.

There were, however, other interesting results that my simple benchmark pointed out. More on that in a moment, but first…

Books, books, books

Dear reader,

You probably know that I write books. A big part of making a book, however, is not just writing it, but letting all the potential readers out there know that the book exists. I'm doing a lot there - and so is my publisher - but still we can't reach all the potential readers ourselves.

That's where you come in!

If you have read any of my books and if you loved it and want the others to know, please consider publishing a review on the Amazon site. More reviews make Amazon algorithms treat the book with more respect and they recommend it more to customers.

Leave the review here: Delphi High Performance, Hands-on Design Patterns with Delphi.

You can also just tell your colleagues in any social circle - digital or real-life - that you like the book. Spread the word!

If you think that is too much and you don't want to get involved so deep, it doesn't matter. I still love you.

On the other hand, if you didn't like my book, let me know. I want to improve, so tell me why you don't like it and what I can do to make my next book better. My contact info is here.

Thank you,

Primož

Deep Dive into Design Patterns

While writing Design Patterns with Delphi, I spent quite some time researching existing literature on design patterns implementation in Delphi, design patterns in other languages, other types of patterns, design principles and so on …

In case you would like to dig deeper than the book takes you, here is my reading list.

Spring conference, spring edition

Hurry up, you only have five (5) days left to get a Spring4D European Conference ticket at the regular price!





It’s gonna be a blast! Stefan will be there, of course (can you imagine Spring 4D without him in the picture?), and so will be I. There will be a ton of interesting sessions, and lots of time for eye-to-eye discussions. Marco has announced his presence, I’ll be bringing my books (or you can bring your own copy, if you already own it and want it signed), Stefan will be accepting musical requests (not really ;) ) …

You can join us for a day or two. Your call, but I would certainly recommend the latter, because this conference will ROCK!

“Bee there Orr Bee A Rectangular Thyng”
- The Band With Rocks In *

Delphi developer needed (Slovenia)

Slovenian company BASS is looking for a Delphi developer (on-site in Celje, Slovenia).

(I’m not affiliated with them; they just asked me to spread a word around. If you have any questions, contact them directly.)