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Sunday, March 21, 2010

Hyperthreading, Win 7 Scheduling and Core Parking, A Simple Tale

I was asked by a pretty savvy engineer friend if I could explain Hyyperthreading to a ten year old. I said "sure, if I can't explain something in a way a bright ten year old can understand it, I probably don't understand it well enough myself." I was faced with similar problems in my engineering days, having to present to the start up money guys that were not technicians.

So, here's my go at it.

Think of it this way. You have a bank, with four tellers (cores). Many customers (threads) waiting to get serviced (CPU time on a core). These tellers are pretty bright, they can each multi-task (hyperthreading) between two customers (threads on each logical core). They can each have two customers (threads) at their window, but can only work with one customer at a time (there is after all only one physical core per two logical cores of the CPU), but if that customer has to fill out a form (the thread stalls, or sleeps, or has used its fair share of CPU time), the teller can work on the next customer (thread) in their line (the two hyperthreads per core). They simply make a note of what they are doing with the first customer (the thread CPU states), put it aside (each core in the CPU has two areas per core to track state), and start working on the second customer (thread). The tellers (cores) can work very efficiently with these two customers (threads), and can do so for as long as the bank manager (Operating System) lets them, because they basically have everything they need to know about those two customers (threads) right at their fingertips (the two areas of each core that hold this information.)

Above all of this is the bank manager (the OS). The manager decides which two customers (threads) are at each teller (core) at any given time, and can swap one of those customers (threads) with another waiting customer (thread) in the bigger line of waiting customers (the whole thread pool for the OS).

Now it so happens that this swapping slows the servicing of the customers (threads), so the manager (OS) avoids this at all costs. In addition, the manager (OS) knows that the more work (threads) he can keep on as few tellers (cores), the better. In fact, if the manager (OS) can, he'll put a teller (core, real or logical) on break (CPU parking), not having to pay them during this time (energy savings for the CPU.) Even more interesting, the manager (OS, actually OS and CPU features) knows that if he can push as much work on to a few tellers (cores), these tellers drink a big cup of coffee and work even faster than normal (turbo-boost). The manager (OS/CPU features) knows that there's not enough coffee to go around to all the tellers (cores), and that all of the tellers (cores) cant all be working at the faster rate, so the manager (OS/CPU features) tries to keep as few tellers (cores) active as possible, so long as he thinks it won't affect the overall servicing of the customers (thread pool).

Playing with things like the manager's decisions (thread scheduling) by overriding him (playing with affinity) can force all of the tellers (cores) to do work, even when the manager (OS/CPU features) would dictate this is not the most efficient way to do things. It will likely have no effect on the rate of servicing of the whole customer collection (thread pool), and may in fact cause all of the tellers (cores) to work at normal speed (no turbo-boost), slowing things down in reality.

The manager (OS) knows best, that's why you hired him. Barring him being drunk (a bug in the OS or CPU scheduling logic), he'll usually make better decisions than you.

Thursday, March 18, 2010

Port Forwarding: Slaying the Mythical Dragon of Online PC Gaming.

Every day, in PC game enthusiast forums around the world, posters having connectivity problems (or not) with their PC game are advised 'You need to forward your ports!', usually by posters claiming to be 'experienced gamers' or 'network experts'.

Most blindly follow this advice, not even understanding what it means to 'forward a port', much less the ramifications of doing so. My intent is to set the record straight for the reader, so that they may better understand the how, what, when, why, and where of port forwarding. I have greatly simplified and generalized  the terminology and examples, which may offend experts, but is appropriate for the intended audience.

To be clear, forwarding of ports is seldom if ever required to allow the client of the online PC game to function properly. Unnecessarily forwarding ports is not only undesirable, it may expose the gamer to security risks, and can interfere with proper functioning of their environment, including games.

The typical PC gamer has a pretty simple environment: Their PC, a router, a modem (perhaps a unit that combines the two functions of router and modem), and...and that's it. The router serves the function of shepherding traffic from the gamer's local area network (LAN) to the wide area network (WAN), where the online game servers 'live'. The modem provides the electronic means for the gamer to access the WAN infrastructure. In some cases, these two functions (router and modem) are combined into a single unit, variously called a router or modem, depending on who you're asking. Often, gamers have a router in their environment without knowing it - they've been told 'that's your modem'.

Why these pieces of hardware are used comes down to the subject of addresses. Each PC in a network must be assigned a unique address. The gamer is probably familiar with these. They're the number sets like '' you might see for you PC on your LAN, or the '' you might see if you ping http://www.google.com/. You've probably heard them called the 'IP address'. The important thing is that each PC must have a unique address. Much like your mail goes to a unique address, if different households could have the same address, you can imagine the mess that would ensue.

Now early in the days of the 'net', the groups defining various standards and protocols decided it would be wise to have addresses that were 'public', that is, known to the world as the address to send to, and 'private', that is, addresses that the 'outside' (WAN) world can't even see. This was done for many reasons including reducing the need for public addresses to be used, and to allow enterprises to split up a 'public' address into one or more internal 'private' addresses.

The router's primary function is to manage, control, and manipulate the barrier between the 'private' LAN and the 'public' WAN.

In a typical environment, the modem provides the connection to the WAN, giving the user on the 'inside' of the modem connection some public IP address on the WAN assigned by their ISP. The router takes the traffic from the PCs on the LAN and passes it on through the modem to the destination server on the WAN. We'll call this the 'request' to the server. The server does whatever it needs to process the request, and responds to the WAN address of the gamer. We'll call this the 'reply' from the server.

The router will keep track of requests sent out to the WAN, and in general only allow traffic from the WAN to a PC on the LAN if it determines that traffic is an appropriate reply from a server to a request from a PC on the LAN. Now the router/modem usually have one, and only one public WAN address assigned to them. What are we to do if we have several PCs on our lan that all want to make requests to the same server on the WAN and get their respective replies? The router does this for us through a mechanism generically called Network Address Translation, or NAT for short. There are many details we won't delve into here, a good overview can be found at http://en.wikipedia.org/wiki/Network_address_translation, with some useful references. Readers that wish a more in depth treatment might use the superb books by Comer at http://www.cs.purdue.edu/homes/dec/.

The problem NAT solves is analogous to sending mail between two apartment buildings. We know the street address where we want to send it (the IP address), and the apartment number. In the IP world, the apartment number is called the 'port'. For our PC game, the game client (what the gamer plays) needs to send requests to the game server(s), and it does so by sending requests to the IP address of the server, and including the port that address should go to on the server. The request needs to have a 'return address' so the server can reply, so the game will add the address of the game client, and the desired return port to the request.

Now as we've said, the client is on a private address. The server can't see this or do anything with it. So the router changes the address information, replacing the private LAN address with its public WAN address, and remembers the return address port for the request. If more than one PC on the LAN make a request to a server and specify the same return port, the router notices this, and changes the return port along with the return IP address, keeping track of which PC corresponds to which requested return port the router sends in the request to the server. When the server replies, it uses the return address of the client, which will be the public IP (WAN) address of the gamer, and the return port, which may have been changed from the actual return port by the router.

When the router sees this traffic, it peeks into the packet and determines which PC belongs to the requested return port. The router changes the return port to the one originally in the PC's request, if needed, changes the return IP address to that of the correct PC, and passes the traffic onto the LAN, where the PC that made the request will receive its reply from the server.

In general, we don't want random traffic coming from the WAN into our LAN. Because the router peeks into traffic to determine if it even belongs on the LAN, random attempts to enter the LAN are thwarted. Unless the user specifically needs to have requests from the WAN enter the lan (to a server of some sort on our LAN to reply to), this is precisely what we desire. Routers usually include some kind of 'firewall' capability, which considerably enhances the security of the client<->server interchanges, and provides even more probative capability toward unsolicited traffic from the WAN. We will not detail firewall functionality.

What if the gamer needs to have a server on the LAN that can be accessed by others on the WAN? How might we accomplish this? This is where the feature of the router called 'port forwarding' comes into play. The user can configure their router, and set it to allow traffic from the WAN to its WAN address into the LAN. The user does this by specifying what PC is going to reply to traffic on which port(s). For example, if we wanted to run our own web server on our LAN (or game server, just change the nomenclature and numbers), it would need to get requests on port 80, the default port number for HTTP (browser) traffic. If the PC running our web server on our LAN had an address of say, we would configure the router to forward any traffic from the WAN to its WAN address with a destination port of 80 to the PC at When the web server (or game server) replies to the request, it is sent through the router back to the WAN address of the original requester. The same kinds of manipulations to the address happen via NAT as with the game client example, just in reverse. So forwarding is for clients on the WAN to get to a server on your LAN. Pretty simple, no?

Now, to kill the dragon!

Modern PC games played online need the game client to make requests to the game server. The game server, and other game clients, do not make requests to the game client. There are exceptions to this, namely some peer-to-peer games, and cases where one of the clients is also running the game server on one of their PCs. Both fall into the generalized description of a server from earlier. But in general, modern games are client-server based, where the server is run by a provider on the WAN, and the gamer plays the client on the LAN. At no time do the servers try to make an 'inbound' request to the client. Hence, forwarding of any ports to play the game is completely unnecessary, and accomplishes nothing. Forwarding ports when not explicitly required poses a security risk to the user, and can in fact interfere with proper traffic flow for games.

The game's client makes the requests, the router handles the manipulation and shepherding of the traffic to the server on the WAN and the corresponding reply traffic from the server on the WAN to the game client on the LAN. Not the other way around!

Unfortunately, 'You need to forward your ports!' is one tough dragon to slay, and this myth is constantly perpetuated in forums, and even by occasionally by misinformed game publisher support staff. There are even whole web sites devoted to the subject, with applications to automate this unnecessary and potentially security compromising router feature for the uninformed user.

Unless you are instructed that your game requires ports to be forwarded from an authoritative source (the game manual, the game developers, or in some cases the publisher with the caveat noted earlier), you are likely not required to do it. Abandon all hope ye that consider enthusiast forums to be an authoritative source!

To humanize it, think of it this way: You, in your household, act as the 'router' and 'firewall' in a way for traffic in and out of your house (your LAN). You, and others in the house are free to go out from the house to seek information (onto the WAN). When someone comes knocking at the door with the answer, you can peek through the peephole on your door and decide if you expected them, and let them into your house. If a stranger comes knocking, you're likely to decide they're uninvited, and not let them in. Port forwarding is giving a stranger the key to your door. In fact, its giving the key to your door to everyone in the world that knows how to get to your door! The 'experienced gamers' and 'net experts' that tell you there's no danger in forwarding ports when it's not explicitly required are doing just that: telling you it's OK to give the key to your front door to everyone on the planet. I'd venture most intelligent readers wold never subscribe to such nonsense.

How many readers in game enthusiast forums do you think blindly forward ports to 'fix' problems? How many of those same readers will download the latest coolest 'tweak tool' for the game when offered up on the forum? How hard do you think it would be to perhaps list some real ports for the game, and throw an extra one in that the later downloaded 'tweak tool' actually listens on, allowing a remote intruder in to the victim's PC to run amok? If you don't know how to verify exactly why a game should need ports forwarded, exactly which ports should be forwarded, and know exactly how to do this, you probably shouldn't. Since port forwarding, with a properly configured and behaving modem/router is not needed by any modern PC game client, you probably shouldn't anyway.

Before ending and in all fairness, it should be noted that some misbehaving or otherwise buggy routers can be 'worked around' by forwarding ports where this would not normally be required. Part of this 'You need to forward your ports!' malarkey is undoubtedly from uniformed users seeing this 'fix' an issue, not understanding that the problem in fact is elsewhere and the 'fix' is a bandage that may cause other problems and security issues. Used properly, this can allow routers that restrict the user to NAT other than Type 1/Cone to mimic a properly behaving full-cone router for a game. This will of course be limited to only one PC on the LAN side and will not allow multiple players to simultaneously play from the LAN if this work-around is needed. See Troubleshooting Multi-Player PC Game Connectivity Issues for examples of this.

I hope after reading this, the reader has a clarified understanding of what port forwarding is, and when its use is appropriate.

Wednesday, March 17, 2010

Troubleshooting Multi-Player PC Game Connectivity Issues

Here lies the original post that started this blog, a troubleshooting guide for PC gamers. This guide provides suggestions for possible solutions to common, and not so common connectivity problems.

The suggestions are presented in a form generalized enough to prevent the document from becoming a book (and it's already plenty long), but not so much so that they become useless.

Using these suggestions with the details provided, along with resources on the web, your hardware and software documentation and Google should you need to do low-level changes where step-by-step details would have been impractical to provide for reasons of document length or where steps vary wildly for differing equipment, you should be able to resolve your connection problems.

Regardless, do note that this is not for the faint of heart: there is a lot of ground to cover and many subtleties regarding PC game connectivity and issues involved in troubleshooting problems.

You can view the current incarnation of this document at:

Please leave any comments and suggestions here, or via e-mail as shown in the document.