Can the thread per request model be faster than non-blocking I/O?_问答_开发者

I remember 2 or 3 years ago reading a couple articles where people claimed that modern threading libraries were getting so good that thread-per-request servers would not only be easier to write than non-blocking servers but that they'd be faster, too. I believe this was even demonstrated in Java with a JVM that mapped Java threads to pthreads (i.e. the Java nio overhead was more than the context-switching overhead).

But now I see all the "cutting edge" servers use开发者_开发百科 asynchronous libraries (Java nio, epoll, even node.js). Does this mean that async won?

Not in my opinion. If both models are well implemented (this is a BIG requirement) I think that the concept of NIO should prevail.

At the heart of a computer are cores. No matter what you do, you cannot parallelize your application more than you have cores. i.e. If you have a 4 core machine, you can ONLY do 4 things at a time (I'm glossing over some details here, but that suffices for this argument).

Expanding on that thought, if you ever have more threads than cores, you have waste. That waste takes two forms. First is the overhead of the extra threads themselves. Second is the time spent switching between threads. Both are probably minor, but they are there.

Ideally, you have a single thread per core, and each of those threads is running at 100% processing speed on their core. Task switching wouldn't occur in the ideal. Of course there is the OS, but if you take a 16 core machine and leave 2-3 threads for the OS, then the remaining 13-14 go towards your app. Those threads can switch what they are doing within your app, like when they are blocked by IO requirements, but don't have to pay that cost at the OS level. Write it right into your app.

An excellent example of this scaling is seen in SEDA http://www.eecs.harvard.edu/~mdw/proj/seda/ . It showed much better scaling under load than a standard thread-per-request model.

My personal experience is with Netty. I had a simple app. I implemented it well in both Tomcat and Netty. Then I load tested it with 100s of concurrent requests (upwards of 800 I believe). Eventually Tomcat slowed to a crawl and exhibited extremely bursty/laggy behavior. Whereas the Netty implementation simply increased response time, but continued with incredibly overall throughput.

Please note, this hinges on solid implementation. NIO is still getting better with time. We are learning how to tune our servers OSes to work better with it as well as how to implement the JVMs to better leverage the OS functionality. I don't think a winner can be declared yet, but I believe NIO will be the eventual winner, and it's doing quite well already.

It is faster as long as there is enough memory.

When there are too many connections, most of which are idle, NIO can save threads therefore save memory, and the system can handle a lot more users than thread-per-connection model.

CPU is not a direct factor here. With NIO, you effectively need to implement a threading model yourself, which is unlikely to be faster than JVM's threads.

In either choice, memory is the ultimate bottleneck. When load increases and memory used approaches max, GC will be very busy, and the system often demonstrate the symptom of 100% CPU.

Some time ago I found rather interesting presentation providing some insight on "why old thread-per-client model is better". There are even measurements. However I'm still thinking it through. In my opinion the best answer to this question is "it depends" because most (if not all) engineering decisions are trade offs.

Like that presentation said - there's speed and there's scalability.

One scenario where thread-per-request will almost certainly be faster than any async solution is when you have a relatively small number of clients (e.g. <100), but each client is very high volume. e.g. a realtime app where no more than 100 clients are sending/generating 500 messages a second each. Thread-per-request model will certainly be more efficient than any async event loop solution. Async scales better when there are many requests/clients because it doesn't waste cycles waiting on many client connections, but when you have few high volume clients with little waiting, it's less efficient.

From what I seen, authors of Node and Netty both recognize that these frameworks are meant to primarily address high volumes/many connections scalability situations, rather than being faster for for a smaller number of high volume clients.