# A Gentle Intro to Libevent

These past two days in some free time, I decided to explore this nifty C library called libevent.

Following the theme from the previous post, the first question is: ‘Why do we need it?’. If you are familiar with network programming, or any multithreaded programming which involves blocking IO, you already know the problem at hand. Right from the hardware level to the software level, a common problem that happens is: IO is slower than the CPU. If we have several tasks to finish, and the current task being executed is waiting for a blocking IO to finish, we should ideally work on the other tasks and let that blocking IO finish in the background, and check on it later.

When we have several such operations happening in the background, we need a way to figure out when a particular operation (such as read, write, accept a connection), can be performed without blocking, so that we can quickly do that, and return to other things. select(2), poll(2), epoll(4), kqueue(2) (on *BSD systems), are one of the several ways to do it. In essence, you register a set of file descriptors that you are interested in, and then call one of these ‘backends’. They would usually block until either one of the fd-s that you are interested in, is ready for data to be read or written to it. If none of them is ready, it would block for a configured amount of time and then return.

The problem that libevent solves is, it provides an easy to use library for notifying when an event happens on the file descriptors which you consider interesting. It also hides the real backend (select, epoll, kqueue) being used, and this helps you avoid writing platform-dependent code (eg., kqueue works only on *BSD) and if there were a new and improved backend in the future, your code would not change. It is like the JVM for asynchronous event notification system.

I only have experience with select, so my context is limited. Using select is very tedious.

In essence, what we do here is to create a set of file descriptors (fd_set). We then run a loop where every time we set all the file descriptors we are interested in, into that set. Then we call select(), and it either times out, or one of the bits in that set would be set. We have to check for each of the file descriptors. This makes it a little ugly. Other backends might be more pleasant to use, but libevent is way ahead of select in terms of usability. Here is some sample code:

An event_base represents a single event loop like the one that we saw in the select example. To subscribe to changes in a file descriptor, we will first create an event. This can be done using event_new, which takes in the event base we created, the file descriptor, flags signalling when the event is active, the callback method and arguments to the callback method. In this particular example, we ask that the acceptConnCob method be called when the file descriptor is ready for reading (EV_READ) and persist this event, i.e, even when the event fires, automatically add it for the next time (EV_PERSIST is to be used here). Note that we had to add the file descriptors in every iteration of the while loop of the select example, so using the EV_PERSIST flag is a slight convenience here. Once, I have created the event, I need to add that event to the event_base it should be processed by, along with a timeout, using the event_add method. If the fd doesn’t become active by the specified time, the callback will be called anyways. Finally, to get things running, we will ‘dispatch’ the event base, which will spawn a new thread to run the event loop.

Note that nowhere have I specified which backend to use. I don’t need to. However, there are ways to prefer or avoid certain backends in libevent, using the event_config struct. Refer to the link in the end.

I can add multiple events, and there are a lot of options that you can use. For instance, I can create a timer by passing -1 as a file descriptor with the EV_TIMEOUT and EV_PERSIST flags, and the required timeout in event_add. This would call the timeout callback every ‘timeout’ seconds. Example:

I created a simple fortune cookie server (one of my favorite demo examples), where I have a set of messages, and if someone asks me for a fortune cookie, I will give them the current fortune cookie. Every few seconds, I will pick a new fortune cookie to be returned. This is implemented by having two events, one for accepting connections and the other for having a timer. The code is here.

One small thing to keep in mind is that if the callbacks themselves to do something heavy, then it defeats the purpose of using libevent. This is because the callbacks are called from the same thread as the actual event loop. The longer the callback runs, the longer the event loop is not able to process other events.

libevent allows you to do a lot of customizations. In the above example, I have added callbacks to override the logging mechanism, so that I can use glog (Google’s logging library). There are several other features such buffered events and a lot of utility methods (such as creating simple http servers), that you can find in the libevent book.

There are other similar async event notification systems such as libev, and libuv, etc. but I haven’t had the time to figure out the differences. I hope to cover the very interesting wrapper around libevent in folly, Facebook’s open source C++ library, in a future post.

# Notes of a Software Engineer - Understand the Problem

This is my first post on this blog, which doesn’t discuss any specific problem / technology. As I have been spending time writing code, I feel that it is a good practice to sit back and introspect once in a while. This post has some thoughts, which I hope will make people sit up and notice if they are making the same mistakes. Because this is a long-ish post, some might not read it completely, or some might not be able to relate to the content right now. Regardless, I feel eventually a significant number of us will make the mistakes that I have made in the past, and learn from it first hand. Whatever the case may be, if this makes any sense to you, please share your thoughts and experiences with me :)

As Engineers I feel we are often excited to work on new and ambitious projects. I am specifically talking about non-trivial projects which break new ground, and/or have a reasonable change of not succeeding. The latter could be because it is often the case that, these projects are complicated enough and its hard to be exact with respect to the benefits. These projects might also touch certain areas of the system which are hazy in general.

‘Hazy’ doesn’t really imply that that particular area / part of the system, is naturally hard to understand. It could be just that we don’t know the problem well enough, and how it interacts with those ‘hazy’ areas. I cannot stress enough that it is critical to understand the problem really well before hand. It seems clichéd, and has been repeated so many times, that it will probably not make a good enough impact. So, I will repeat this again in detail, so it stays with you and me, a little longer.

## Understand The Problem

As per Prof. Bender, when giving a presentation, making sure that people understand why we did what we did, is the most important thing. Extending this backwards, ever wondered if that problem really needs to be solved in the first place? A lot of times, as a new CS graduate, working on my first full-time unsupervised big tasks, I would really be in awe of the supposed problem. Looking with rose-tinted glasses, you feel that this is what you had told the recruiter and interviewers that you wanted to do in the job. Excellent, lets start working on it. And if you do this, and just jump into this directly, you are going to have a bad time.

Often I did not spend enough time understanding why exactly was I doing what I was doing. Do benchmarks show that this is really needed? Do I have a good enough prototype which shows that if I do what I am going to do, it will give us significant benefits? Do people need this? Has this problem been solved before? What is the minimum I can do to solve this reasonably, and move on to other bigger problems?

This proactive research is what I feel is the difference between new and experienced engineers. In fact, I think, in some cases senior engineers write LESS code than the less experienced ones and still get more things done. Its now clear to me, that the actual coding should only take 10% of the time allocated to the project. If I spend enough time doing my due-diligence and am ‘lazy’, I can simply prune some potential duds much before they turn into huge time sinks. If I spend some more time on the problems which actually require my time, I can figure out things I can do to reduce the scope of the problem, or cleverly use pre-built solutions to do part/most of the work. All this can only come if we (and I will repeat again) Understand. The. Problem.

(Please let me know if you agree or disagree with me about what I said. I would love to hear back).

[0] Sloth picture courtesy: http://en.wikipedia.org/wiki/File:SlothDWA.jpg

# Make an Amount N With C Coins

This week we planned to discuss Dynamic Programming. The idea was to discuss about 4-5 problems, however, the very first problem kept us busy for an entire hour. The problem is a well known one: ‘Given an infinite supply of a set of coin denominations, $C = {c_1, c_2, c_3, …}$, in how many ways can you make an amount N from those coins?’

The first question to be asked is, whether we allow permutations? That is, if, $c_1 + c_2 = N$, is one way, then do we count $c_2 + c_1 = N$, as another way? It makes sense to not allow permutations, and count them all as one. For example, if $N$ = 5, and $C$ = {1, 2, 5}, you can make 5 in the following ways: {1, 1, 1, 1, 1}, {1, 1, 1, 2}, {1, 2, 2}, {5}.

We came up with a simple bottom-up DP. I have written a top-down DP here, since it will align better with the next iteration. The idea was, $f(N) = \sum f(N-c[i])$ for all valid $c[i]$, i.e., the number of ways you can construct $f(5) = f(5-1) + f(5-2) + f(5-5) \implies f(4) + f(3) + f(0)$. $f(0)$ is 1, because you can make $0$ in only one way, by not using any coins (there was a debate as to why $f(0)$ is not 0). With memoisation, this algorithm is $O(NC)$, with $O(N)$ space.

This looks intuitive and correct, but unfortunately it is wrong. Hat tip to Vishwas for pointing out that the answers were wrong, or we would have moved to another problem. See if you can spot the problem before reading ahead.

The problem in the code is, we will count permutations multiple times, for example, for $n = 3$, the result is 3 ({1, 1, 1}, {1, 2} and {2, 1}). {1, 2} and {2, 1} are being treated distinctly. This is not correct. A generic visualization follows.

Assume we start with an amount $n$. We have only two types of coins of worth $1$ and $2$ each. Now, notice, how the recursion tree would form. If we take the coin with denomination $1$ first and the one with denomination $2$ second, we get to a subtree with amount $n-3$, and on the other side, if we take $2$ first, and $1$ next, we get a subtree with the same amount. Both of these would be counted twice with the above solution, even though, the order of the coins does not matter.

After some discussion, we agreed on a top-down DP which keeps track of which coins to use, and avoids duplication. The idea is to always follow a lexicographic sequence when using the coins. It doesn’t matter if the coins are sorted or not (actually yes, if you check all the coins that you are allowed to use, if they can be used). What matters is, always follow the same sequence. For example, if I have three coins {1, 2, 5}. Let’s say, if I have used coin $i$, I can only use coins $[i, n]$ from now on. So, if I have used coin with value $2$, I can only use $2$ and $5$ in the next steps. The moment I use 5, I can’t use 2 any more.

If you follow, this will allow sequences of coins, in which the coin indices are monotonically increasing, i.e., we won’t encounter a scenario such as {1, 2, 1}. This was done in a top-down DP as follows:

Now, this is a fairly standard problem. I decided to check on the interwebs, if my DP skills have been rusty. I found the solution to the same problem on Geeks-for-Geeks, where they present the solution in bottom-up DP fashion. There is also an $O(N)$ space solution in the end, which is very similar to our first faulty solution, with a key difference that the two loops are exchanged. That is we loop over coins in the outer-loop and loop over amount in the inner loop.

This is almost magical. Changing the order of the loops fixes the problem. I have worked out the table here step by step. Please let me know if there is a mistake.

Step 1: Calculating with 3 coins, uptil N = 10. Although we use an one-dimensional array, I have added multiple rows, to show how the values change over the iterations.

Step 2: Initialize table[0] = 1.

Step 3: Now, we start with coin 1. Only cell 0 has a value. We start filling in values for $n = 1, 2, 3, ..$. Since, all of these can be made by adding \$1 to the amount once less then that amount. Thus, the total number of ways right now, would be 1 for all, since we are using only the first coin, and the only way to construct an amount would be$1 + 1 + 1 + … = n$. Step 4: Now, we will use coin 2 with denomination \$2. We will start with $n = 2$, since we can’t construct any amount less than \$2 with this coin. Now, the number of ways for making amount \$2 and \$3 would be$2$. One would be the current number of ways, the other would be removing the last two$1$s, and adding a two. Similarily, mentally (or manually, on paper) verify how the answers would be. Step 5: We repeat the same for 3. The cells with a dark green bottom are the final values. All others would have been overwritten. I was looking into where exactly are we maintaining the monotonically increasing order that we wanted in the top-down DP in this solution. It is very subtle, and can be understood, if you verify step 4 on paper, for$4, 5, 6, …$and see the chains that they form. In the faulty solution, when we compute amounts in the outer loop, when we reach to amount$n$, we have computed all previous amounts for all possible coins. Now, if you compute from the previous solutions, they have included the counts for all coins. If you try to calculate the count for$n$, using the coin$i$, and result for$n - cval[i]$, it is possible, that the result for$n - cval[i]$, includes the ways with coins >$i$. This is undesirable. However, when we compute the other way round, we compute for each coin at a time, in that same lexicographical order. So, if we are using the results for$n - cval[i]$, we are sure, that it does not include the count for coins >$i$, because they haven’t been computed yet, since they would only happen after computing the result for$i\$.

As they say, sometimes being simple is the hardest thing to do. This was a simple problem, but it still taught me a lot.