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Boosting Socket Performance on Linux
Cop writes "The Sockets API lets you develop client and server applications that can communicate across a local network or across the world via the Internet. Like any API, you can use the Sockets API in ways that promote high performance -- or inhibit it. This article explores four ways to use the Sockets API to squeeze the greatest performance out your application and to tune the GNU/Linux® environment to achieve the best results."
Some engineers at Berkeley have been looking at this for a while, but haven't gotten much credit for it.
Judging by the response time from IBM's web server, it looks like they have yet to put their advice into practice.
I mean really, I think we understand what you mean by just saying Linux.
If we don't fight for ourselves no one will.
This reads like an article from the 90's. This being 2006 and all, I would hope that programmers know how to make effective use of TCP/IP sockets. I wonder if maybe they just yanked an article from 1995 and did a search/replace on s/Windows/GNU Linux/g.
Javascript + Nintendo DSi = DSiCade
Here is the summary:
The Sockets API lets you develop client and server applications that can communicate across a local network or across the world via the Internet. Like any API, you can use the Sockets API in ways that promote high performance -- or inhibit it. This article explores four ways to use the Sockets API to squeeze the greatest performance out your application and to tune the GNU/Linux® environment to achieve the best results.
Here is the first paragraph of the article:
The Sockets API lets you develop client and server applications that can communicate across a local network or across the world via the Internet. Like any API, you can use the Sockets API in ways that promote high performance -- or inhibit it. This article explores four ways to use the Sockets API to squeeze the greatest performance out your application and to tune the GNU/Linux® environment to achieve the best results.
Unless Cop (the submitter) is actually M. Tim Jones (the article author), Cop didn't write a darn thing.
Didn't we just have this discussion on
I want to drag this out as long as possible. Bring me my protractor.
Tuning socket parameters is great and all, but the real performance problem with socket IO has to do with using select and poll. There are high-performance alternatives (which admittedly tend to vary from OS to OS) that are so far superior that I wouldn't even consider the default methods unless complete code portability were a crucial factor.
...on developerWorks, not the least of which, if I may say so, is the GLib tutorial I wrote for them this past summer. If you wanted how to use various GLib collections and utilities - lists, tables, trees, quarks, relations, and all that - check it out. You can even download a nice PDF file for offline perusing.
Folks who are thinking about writing something technical - give dW a shot. The editors are savvy folks and there's lots of good stuff up there already.
Oh, and book plug!
The Army reading list
Most probably it's just IBM policy to always acknowledge some one else's trademarks, so as not to get in trouble. Both GNU (yeah, I know! I knooow..) and Linux are registered trademarks (... of their respective owners, of course..)
SCO employee? Check out the bounty
Is that what you're looking for?
There was a Boost library in the works to encapsulate all of this rather nicely, but I'm not sure if it ever made it out of beta. ACE is another option, though that tends to be overkill for some projects. I rolled my own class wrapper around this stuff, but then I enjoy library programming.
To get around the above problems, I came up with the following scheme: Leave Nagle's algorithm enabled, but create a FlushSocket() function that merely disables Nagle on the socket, then calls send() on the socket with a 0-byte buffer, then enables Nagle again. This apparently forces the TCP stack to immediately send any data that it may have accumulated in its Nagle-buffer. Therefore the only thing the calling code has to remember to do is to call FlushSocket() whenever it has called send() one or more times and doesn't think it will be sending any more data any time soon.
The above technique seems to work pretty well under Linux, Windows, and OS/X (and is more portable than Linux-specific flags like TCP_CORK, etc), but I haven't seen it documented anywhere. Is that simply an oversight, or is there some nasty downside to this technique that I'm overlooking?
I don't care if it's 90,000 hectares. That lake was not my doing.
It ignores you except at feeding time, and pees in your shoes when it's mad at you?
throughput = window_size / RTT
110KB / 0.050 = 2.2MBps
If instead you use the window size calculated above, you get a whopping 31.25MBps, as shown here:
625KB / 0.050 = 31.25MBps
That's funny, I get 12.5MBps
???
Documentation like this is great and extremely valuable. It would be much more valuable, however, if it remained current. For example, can the ABISS project (which improves block I/O) be used at all? What do the numbers look like, when using profiling tools like Web100 (which profiles TCP communications)?
Has anyone run the Linux or one of the *BSD kernels through DAKOTA, KOJAK or PAPI to determine where, precisely, bottlenecks are within the kernels? It's easy to theorise, but isn't it cleaner to measure?
Now, I'm not saying these things aren't being done. They probably are, somewhere, by someone, but if the results aren't getting published we don't really know what impact what changes are going to have. The current method of evolving Operating System code in general is often a mix of personal theory and subjective experience based on non-random samples of activity. That can't really be a good way to do things, can it?
If I'm wrong, feel free to say. If I'm right, then maybe it would be a good thing if someone (possibly me) put together some kind of testing kit for measuring Linux kernel performance and actually measured the stats for Linux kernels on some kind of regular basis.
It's a small world and it smells funny; I'd buy another if it wasn't for the money; Take back what I paid (SoM)
I don't know if that really makes sense for networking though... the reason you'd want fflush() to block until the data makes it to disk is so that once your call to fflush() returns you know that your written data is safe in the event of a crash or power failure. (Although with too-clever hard drive firmware I'm not so sure even that's true anymore!). With networking on the other hand, even once the data has left your Ethernet port there is no guarantee that it will get to its destination... so what would be the purpose is waiting?
I don't care if it's 90,000 hectares. That lake was not my doing.
Here's the real problem, and its solution.
The concept behind delayed ACKs is to bet, when receiving some data from the net, that the local application will send a reply very soon. So there's no need to send an ACK immediately; the ACK can be piggybacked on the next data going the other way. If that doesn't happen, after a 500ms delay, an ACK is sent anyway.
The concept behind the Nagle algorithm is that if the sender is doing very tiny writes (like single bytes, from Telnet), there's no reason to have more than one packet outstanding on the connection. This prevents slow links from choking with huge numbers of outstanding tinygrams.
Both are reasonable. But they interact badly in the case where an application does two or more small writes to a socket, then waits for a reply. (X-Windows is notorious for this.) When an application does that, the first write results in an immediate packet send. The second write is held up until the first is acknowledged. But because of the delayed ACK strategy, that acknowledgement is held up for 500ms. This adds 500ms of latency to the transaction, even on a LAN.
The real problem is that 500ms unconditional delay. (Why 500ms? That was a reasonable response time for a time-sharing system of the 1980s.) As mentioned above, delaying an ACK is a bet that the local application will reply to the data just received. Some apps, like character echo in Telnet servers, do respond every time. Others, like X-Windows "clients" (really servers, but X is backwards about this), only reply some of the time.
TCP has no strategy to decide whether it's winning or losing those bets. That's the real problem.
The right answer is that TCP should keep track of whether delayed ACKs are "winning" or "losing". A "win" is when, before the 500ms timer runs out, the application replies. Any needed ACK is then coalesced with the next outgoing data packet. A "lose" is when the 500ms timer runs out and the delayed ACK has to be sent anyway. There should be a counter in TCP, incremented on "wins", and reset to 0 on "loses". Only when the counter exceeds some number (5 or so), should ACKs be delayed. That would eliminate the problem automatically, and the need to turn the "Nagle algorithm" on and off.
So that's the proper fix, at the TCP internals level. But I haven't done TCP internals in years, and really don't want to get back into that. If anyone is working on TCP internals for Linux today, I can be reached at the e-mail address above. This really should be fixed, since it's been annoying people for 20 years and it's not a tough thing to fix.
The user-level solution is to avoid write-write-read sequences on sockets. write-read-write-read is fine. write-write-write is fine. But write-write-read is a killer. So, if you can, buffer up your little writes to TCP and send them all at once. Using the standard UNIX I/O package and flushing write before each read usually works.
John Nagle