Shallow Thoughts : tags : performance

Akkana's Musings on Open Source Computing, Science, and Nature.

Wed, 13 Nov 2013

Does scrolling output make a program slower? Followup.

Last week I wrote about some tests I'd made to answer the question Does scrolling output make a program slower? My test showed that when running a program that generates lots of output, like an rsync -av, the rsync process will slow way down as it waits for all that output to scroll across whatever terminal client you're using. Hiding the terminal helps a lot if it's an xterm or a Linux console, but doesn't help much with gnome-terminal.

A couple of people asked in the comments about the actual source of the slowdown. Is the original process -- the rsync, or my test script, that's actually producing all that output -- actually blocking waiting for the terminal? Or is it just that the CPU is so busy doing all that font rendering that it has no time to devote to the original program, and that's why it's so much slower?

I found pingu on IRC (thanks to JanC) and the group had a very interesting discussion, during which I ran a series of additional tests.

In the end, I'm convinced that CPU allocation to the original process is not the issue, and that output is indeed blocked waiting for the terminal to display the output. Here's why.

First, I installed a couple of performance meters and looked at the CPU load while rendering. With conky, CPU use went up equally (about 35-40%) on both CPU cores while the test was running. But that didn't tell me anything about which processes were getting all that CPU.

htop was more useful. It showed X first among CPU users, xterm second, and my test script third. However, the test script never got more than 10% of the total CPU during the test; X and xterm took up nearly all the remaining CPU.

Even with the xterm hidden, X and xterm were the top two CPU users. But this time the script, at number 3, got around 30% of the CPU rather than 10%. That still doesn't seem like it could account for the huge difference in speed (the test ran about 7 times faster with xterm hidden); but it's interesting to know that even a hidden xterm will take up that much CPU.

It was also suggested that I try running it to /dev/null, something I definitely should have thought to try before. The test took .55 seconds with its output redirected to /dev/null, and .57 seconds redirected to a file on disk (of course, the kernel would have been buffering, so there was no disk wait involved). For comparison, the test had taken 56 seconds with xterm visible and scrolling, and 8 seconds with xterm hidden.

I also spent a lot of time experimenting with sleeping for various amounts of time between printed lines. With time.sleep(.0001) and xterm visible, the test took 104.71 seconds. With xterm shaded and the same sleep, it took 98.36 seconds, only 6 seconds faster. Redirected to /dev/null but with a .0001 sleep, it took 97.44 sec.

I think this argues for the blocking theory rather than the CPU-bound one: the argument being that the sleep gives the program a chance to wait for the output rather than blocking the whole time. If you figure it's CPU bound, I'm not sure how you'd explain the result.

But a .0001 second sleep probably isn't very accurate anyway -- we were all skeptical that Linux can manage sleep times that small. So I made another set of tests, with a .001 second sleep every 10 lines of output. The results: 65.05 with xterm visible; 63.36 with xterm hidden; 57.12 to /dev/null. That's with a total of 50 seconds of sleeping included (my test prints 500000 lines). So with all that CPU still going toward font rendering, the visible-xterm case still only took 7 seconds longer than the /dev/null case. I think this argues even more strongly that the original test, without the sleep, is blocking, not CPU bound.

But then I realized what the ultimate test should be. What happens when I run the test over an ssh connection, with xterm and X running on my local machine but the actual script running on the remote machine?

The remote machine I used for the ssh tests was a little slower than the machine I used to run the other tests, but that probably doesn't make much difference to the results.

The results? 60.29 sec printing over ssh (LAN) to a visible xterm; 7.24 sec doing the same thing with xterm hidden. Fairly similar to what I'd seen before when the test, xterm and X were all running on the same machine.

Interestingly, the ssh process during the test took 7% of my CPU, almost as much as the python script was getting before, just to transfer all the output lines so xterm could display them.

So I'm convinced now that the performance bottleneck has nothing to do with the process being CPU bound and having all its CPU sucked away by rendering the output, and that the bottleneck is in the process being blocked in writing its output while waiting for the terminal to catch up.

I'd be interested it hear further comments -- are there other interpretations of the results besides mine? I'm also happy to run further tests.

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[ 16:19 Nov 13, 2013    More linux | permalink to this entry | comments ]

Fri, 08 Nov 2013

Does scrolling output make a program slower?

While watching my rsync -av messages scroll by during a big backup, I wondered, as I often have, whether that -v (verbose) flag was slowing my backup down.

In other words: when you run a program that prints lots of output, so there's so much output the terminal can't display it all in real-time -- like an rsync -v on lots of small files -- does the program wait ("block") while the terminal catches up?

And if the program does block, can you speed up your backup by hiding the terminal, either by switching to another desktop, or by iconifying or shading the terminal window so it's not visible? Is there any difference among the different ways of hiding the terminal, like switching desktops, iconifying and shading?

Since I've never seen a discussion of that, I decided to test it myself. I wrote a very simple Python program:

import time

start = time.time()

for i in xrange(500000):
    print "Now we have printed", i, "relatively long lines to stdout."

print time.time() - start, "seconds to print", i, "lines."

I ran it under various combinations of visible and invisible terminal. The results were striking. These are rounded to the nearest tenth of a second, in most cases the average of several runs:

Terminal type Seconds
xterm, visible 56.0
xterm, other desktop 8.0
xterm, shaded 8.5
xterm, iconified 8.0
Linux framebuffer, visible 179.1
Linux framebuffer, hidden 3.7
gnome-terminal, visible 56.9
gnome-terminal, other desktop 56.7
gnome-terminal, iconified 56.7
gnome-terminal, shaded 43.8

Discussion:

First, the answer to the original question is clear. If I'm displaying output in an xterm, then hiding it in any way will make a huge difference in how long the program takes to complete.

On the other hand, if you use gnome-terminal instead of xterm, hiding your terminal window won't make much difference. Gnome-terminal is nearly as fast as xterm when it's displaying; but it apparently lacks xterm's smarts about not doing that work when it's hidden. If you use gnome-terminal, you don't get much benefit out of hiding it.

I was surprised how slow the Linux console was (I'm using the framebuffer in the Debian 3.2.0-4-686-pae on Intel graphics). But it's easy to see where that time is going when you watch the output: in xterm, you see lots of blank space as xterm skips drawing lines trying to keep up with the program's output. The framebuffer doesn't do that: it prints and scrolls every line, no matter how far behind it gets.

But equally interesting is how much faster the framebuffer is when it's not visible. (I typed Ctrl-alt-F2, logged in, ran the program, then typed Ctrl-alt-F7 to go back to X while the program ran.) Obviously xterm is doing some background processing that the framebuffer console doesn't need to do. The absolute time difference, less than four seconds, is too small to worry about, but it's interesting anyway.

I would have liked to try it my test a base Linux console, with no framebuffer, but figuring out how to get a distro kernel out of framebuffer mode was a bigger project than I wanted to tackle that afternoon.

I should mention that I wasn't super-scientific about these tests. I avoided doing any heavy work on the machine while the tests were running, but I was still doing light editing (like this article), reading mail and running xchat. The times for multiple runs were quite consistent, so I don't think my light system activity affected the results much.

So there you have it. If you're running an output-intensive program like rsync -av and you care how fast it runs, use either xterm or the console, and leave it hidden most of the time.

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[ 14:17 Nov 08, 2013    More linux | permalink to this entry | comments ]

Fri, 11 Sep 2009

Article: Linux on Multicore Processors

Linux Planet requested an article on multicore processors and how to make the most of them. Happily, I've been playing with that anyway lately, so I was happy to oblige:

Get the Most Out of Your Multicore Processor: Two heads are better than one!

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[ 21:59 Sep 11, 2009    More writing | permalink to this entry | comments ]

Thu, 27 Aug 2009

Linux Bloat 102

Part 2 of my Linux bloat article looks at information you can get from the kernel via some useful files in /proc, at three scripts that display that info, and also at how to use exmap, an app and kernel module that shows you a lot more about what resources your apps are using. How Do You Really Measure Linux Bloat?

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[ 19:52 Aug 27, 2009    More writing | permalink to this entry | comments ]

Thu, 13 Aug 2009

Linux Bloat 101

Continuing my Linux Planet series on Linux performance monitoring, the latest article looks at bloat and how you can measure it: Finding and Trimming Linux Bloat.

This one just covers the basics. The followup article, in two weeks, will dive into more detail on how to analyze what resources programs are really using.

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[ 10:27 Aug 13, 2009    More writing | permalink to this entry | comments ]

Tue, 21 Jul 2009

Tracking down performance hogs

It's been a day -- or week, month -- of performance monitoring.

I'm posting this while sitting in an excellent OSCON tutorial on Linux System and Network Performance Monitoring, by Darren Hoch. It's full of great information and I'm sure his web site is equally useful.

And it's a great extension to topic that's been occupying me over the past few months: performance tracking to slim down software that might be slowing a Linux system down. That's the topic of one of my two OSCON talks this Wednesday: "Featherweight Linux: How to turn a netbook or older laptop into a Ferrari." Although I don't go into anywhere near the detail Darren does, a lot of the principles are the same, and I know I'll find a use for a lot of his techniques. The talk also includes a free bonus tourist tip for San Jose visitors.

Today's Linux Planet article is related to my Featherweight talk: What's Bogging Down Your Linux PC? Tracking Down Resource Hogs. Usually they publish my articles on Thursdays, but I asked for an early release since it's related to tomorrow's talk.

For anyone at OSCON in San Jose, I hope you can come to Featherweight late Wednesday afternoon, or to my other talk, Wednesday just after lunch, "Bug Fixing for Everyone (even non-programmers!)" where I'll go over the steps programmers use while fixing bugs, and show that anyone can fix simple bugs even without any prior knowledge of programming.

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[ 10:58 Jul 21, 2009    More conferences | permalink to this entry | comments ]

Thu, 02 Jul 2009

Disabling mouse/keyboard wakeup

Suspend (sleep) works very well on the dual-Atom desktop. The only problem with it is that the mouse or keyboard wake it up. I don't mind the keyboard, but the mouse is quite sensitive, so a breeze through the window or a big truck driving by on the street can jiggle the mouse and wake the machine when I'm away.

I've been through all the BIOS screens looking for a setting to flip, but there's nothing there. Some web searching told me that under Windows, there's a setting you can change that will affect this, but I couldn't find anything similar for Linux, until finally drc clued me in to /proc/acpi/wakeup.

cat /proc/acpi/wakeup
will tell you all the events that can cause your machine to wake up from various sleep states.

Unfortunately, they're also obscurely coded. Here are mine:

Device  S-state   Status   Sysfs node
SLPB      S4    *enabled  
P32       S4     disabled  pci:0000:00:1e.0
UAR1      S4     enabled   pnp:00:0a
PEX0      S4     disabled  pci:0000:00:1c.0
PEX1      S4     disabled  
PEX2      S4     disabled  pci:0000:00:1c.2
PEX3      S4     disabled  pci:0000:00:1c.3
PEX4      S4     disabled  
PEX5      S4     disabled  
UHC1      S3     disabled  pci:0000:00:1d.0
UHC2      S3     disabled  pci:0000:00:1d.1
UHC3      S3     disabled  pci:0000:00:1d.2
UHC4      S3     disabled  pci:0000:00:1d.3
EHCI      S3     disabled  pci:0000:00:1d.7
AC9M      S4     disabled  
AZAL      S4     disabled  pci:0000:00:1b.0

What do all those symbols mean? I have no clue. Apparently the codes come from the BIOS's DSDT code, and since it varies from board to board, nobody has published tables of likely translations.

The only two wakeups that were enabled for me were SLPB and UAR1. SLPB apparently stands for SLeeP Button, and Rik suggested UAR probably stood for Universal Asynchronous Receiver (the more familiar term UART both receives and Transmits.) Some of the other devices in the list can possibly be identified by comparing their pci: codes against lspci, but not those two.

Time for some experimentation. You can toggle any of these by writing to the wakeup device:

echo UAR1 >/proc/acpi/wakeup

It turned out that to disable mouse and keyboard wakeup, I had to disable both SLPB and UAR1. With both disabled, the machine wakes up when I press the power button. (What the SLeeP Button is, if it's not the power button, I don't know.)

My mouse and keyboard are PS/2. For a USB mouse and keyboard, look for something like USB0, UHC0, USB1.

The UAR1 setting is remembered even across boots: there's no need to do anything to make sure the setting is remembered. But the SLPB setting resets every time I boot. So I edited /etc/rc.local and added this line:

echo SLPB >/proc/acpi/wakeup

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[ 09:21 Jul 02, 2009    More linux/kernel | permalink to this entry | comments ]

Sun, 11 Jan 2009

Cleaning up Firefox "places" history

Firefox decided, some time ago, that whenever I try to type in a local file pathname, as soon as I start typing /home/... I must be looking for one specific file: an article I wrote over two months ago and am long since done with. Usually it happens when I'm trying to preview a new article. I no longer have any interest in my local copy of that old article; it's not bookmarked or anything like that; I haven't viewed it in quite some time. But try to tell Firefox that. It's convinced that the old one (why that one, and not one of the more recent ones?) is what I want.

A recursive grep in ~/.mozilla/firefox showed that the only reference to the old unwanted file was in the binary file places.sqlite.

My places.sqlite was 11Mb. I look through the Prefs window showed that the default setting was to store history for minimum of 90 days. That seemed rather excessive, so I reduced it drastically. But that didn't reduce the size of the file any, nor did it banish the spurious URLbar suggestion when I typed /home/....

After some discussion with folks on IRC, it developed that Firefox may never actually reduce the size of the places.sqlite file at all. Even if it did reduce the amount of data in the file (which it's not clear it does), it never tells sqlite to compact the file to use less space. Apparently there was some work on that about a year ago, but it was slow and unreliable and they never got it working, and eventually gave up on it.

You can run an sqlite compaction by hand (make sure to exit your running firefox first!):

sqlite3 places.sqlite vacuum

But vacuuming really didn't help much. It reduced the size of the file from 11 to 8.8 Mb (after reducing the number of days firefox was supposed to store to less than a third of the original size) and it didn't get rid of that spurious suggestion.

So the only remaining option seemed to be to remove the file. It stores both history and bookmarks, so it's best to back up bookmarks before removing it. I backed up bookmarks to the .json format firefox likes to use for backups, and also exported them to a more human (and browser) readable bookmarks.html. Then I removed the places.sqlite file.

Success! The spurious recommendation was gone. Typing seems faster too (less of those freezes while the "awesomebar" searches through its list of recommendations).

So I guess firefox can't be trusted to clean up after itself, and users who care have to do that manually. It remains to be seen how much the file will grow now. I expect periodic vacuumings or removals will still be warranted if I don't want a huge file; but it's pretty easy to do, and firefox found the bookmarks backup and reloaded them without any extra work on my part.

In the meantime, I made a new bookmark -- hidden in my bookmarklets menu so it doesn't clutter the main bookmarks menu -- to the directory where I preview articles I'm writing. That ought to help a bit with future URLbar suggestions.

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[ 11:00 Jan 11, 2009    More tech/web | permalink to this entry | comments ]

Fri, 31 Oct 2008

GIMP Drag-n-Drop and Open Location without gvfs

Quite a while ago I noticed that drag-n-drop of images from Firefox had stopped working for me in GIMP's trunk builds (2.6 and 2.7); it failed with a "file not found" error. Opening URIs with Open Location also failed in the same way.

Since I don't run a gnome desktop, I assumed it probably had something to do with requiring gnome-vfs services that I don't have. But yesterday I finally got some time to chase it down with help from various folk on #gimp.

I had libgnomevfs (and its associated dev package) installed on my Ubuntu Hardy machine, but I didn't have gvfs. It was suggested that I install the gfvs-backends package. I tried that, but it didn't help; apparently gvfs requires not just libgvfs and gvfs-backends, but also running a new daemon, gvfsd. Finding an alternative was starting to sound appealing.

Turns out gimp now has three compile-time configure options related to opening URIs:

  --without-gvfs          build without GIO/GVfs support
  --without-gnomevfs      build without gnomevfs support
  --without-libcurl       build without curl support

These correspond to four URI-getting methods in the source, in plug-ins/file-uri:

GIMP can degrade from gvfs to gnomevfs to libcurl to wget, but only at compile time, not at runtime: only one of the four is built.

On my desktop machine, --without-gvfs was all I needed. Even without running the gnome desktop, the gnomevfs front-end seems to work fine. But it's good to know about the other options too, in case I need to make a non-gnomevfs version to run on the laptop or other lightweight machines.

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[ 11:09 Oct 31, 2008    More gimp | permalink to this entry | comments ]

Sat, 16 Aug 2008

Fast Pixel Ops in GIMP-Python

Last night Joao and I were on IRC helping someone who was learning to write gimp plug-ins. We got to talking about pixel operations and how to do them in Python. I offered my arclayer.py as an example of using pixel regions in gimp, but added that C is a lot faster for pixel operations. I wondered if reading directly from the tiles (then writing to a pixel region) might be faster.

But Joao knew a still faster way. As I understand it, one major reason Python is slow at pixel region operations compared to a C plug-in is that Python only writes to the region one pixel at a time, while C can write batches of pixels by row, column, etc. But it turns out you can grab a whole pixel region into a Python array, manipulate it as an array then write the whole array back to the region. He thought this would probably be quite a bit faster than writing to the pixel region for every pixel.

He showed me how to change the arclayer.py code to use arrays, and I tried it on a few test layers. Was it faster? I made a test I knew would take a long time in arclayer, a line of text about 1500 pixels wide. Tested it in the old arclayer; it took just over a minute to calculate the arc. Then I tried Joao's array version: timing with my wristwatch stopwatch, I call it about 1.7 seconds. Wow! That might be faster than the C version.

The updated, fast version (0.3) of arclayer.py is on my arclayer page.

If you just want the trick to using arrays, here it is:

from array import array

[ ... setting up ... ]
        # initialize the regions and get their contents into arrays:
        srcRgn = layer.get_pixel_rgn(0, 0, srcWidth, srcHeight,
                                     False, False)
        src_pixels = array("B", srcRgn[0:srcWidth, 0:srcHeight])

        dstRgn = destDrawable.get_pixel_rgn(0, 0, newWidth, newHeight,
                                            True, True)
        p_size = len(srcRgn[0,0])               
        dest_pixels = array("B", "\x00" * (newWidth * newHeight * p_size))

[ ... then inside the loop over x and y ... ]
                        src_pos = (x + srcWidth * y) * p_size
                        dest_pos = (newx + newWidth * newy) * p_size
                        
                        newval = src_pixels[src_pos: src_pos + p_size]
                        dest_pixels[dest_pos : dest_pos + p_size] = newval

[ ... when the loop is all finished ... ]
        # Copy the whole array back to the pixel region:
        dstRgn[0:newWidth, 0:newHeight] = dest_pixels.tostring() 

Good stuff!

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[ 21:02 Aug 16, 2008    More gimp | permalink to this entry | comments ]

Sat, 20 Oct 2007

Firefox, caching, and fast Back/Forward buttons

I remember a few years ago the Mozilla folks were making a lot of noise about the "blazingly fast Back/Forward" that was coming up in the (then) next version of Firefox. The idea was that the layout engine was going to remember how the page was laid out (technically, there would be a "frame cache" as opposed to the normal cache which only remembers the HTML of the page). So when you click the Back button, Firefox would remember everything it knew about that page -- it wouldn't have to parse the HTML again or figure out how to lay out all those tables and images, it would just instantly display what the page looked like last time.

Time passed ... and Back/Forward didn't get faster. In fact, they got a lot slower. The "Blazingly Fast Back" code did get checked in (here's how to enable it) but somehow it never seemed to make any difference.

The problem, it turns out, is that the landing of bug 101832 added code to respect a couple of HTTP Cache-Control header settings, no-store and no-cache. There's also a third cache control header, must-revalidate, which is similar (the difference among the three settings is fairly subtle, and Firefox seems to treat them pretty much the same way).

Translated, that means that web servers, when they send you a page, can send some information along with the page that asks the browser "Please don't keep a local copy of this page -- any time you want it again, go back to the web and get a new copy."

There are pages for which this makes sense. Consider a secure bank site. You log in, you do your banking, you view your balance and other details, you log out and go to lunch ... then someone else comes by and clicks Back on your browser and can now see all those bank pages you were just viewing. That's why banks like to set no-cache headers.

But those are secure pages (https, not http). There are probably reasons for some non-secure pages to use no-cache or no-store ... um ... I can't think of any offhand, but I'm sure there are some.

But for most pages it's just silly. If I click Back, why wouldn't I want to go back to the exact same page I was just looking at? Why would I want to wait for it to reload everything from the server?

The problem is that modern Content Management Systems (CMSes) almost always set one or more of these headers. Consider the Linux.conf.au site. Linx.conf.au is one of the most clueful, geeky conferences around. Yet the software running their site sets

  Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
  Pragma: no-cache
on every page. I'm sure this isn't intentional -- it makes no sense for a bunch of basically static pages showing information about a conference several months away. Drupal, the CMS used by LinuxChix sets Cache-Control: must-revalidate -- again, pointless. All it does is make you afraid to click on links because then if you want to go Back it'll take forever. (I asked some Drupal folks about this and they said it could be changed with drupal_set_header).

(By the way, you can check the http headers on any page with: wget -S -O /dev/null http://... or, if you have curl, curl --head http://...)

Here's an excellent summary of the options in an Opera developer's blog, explaining why the way Firefox handle caching is not only unfriendly to the user, but also wrong according to the specs. (Darn it, reading sensible articles like that make me wish I wasn't so deeply invested in Mozilla technology -- Opera cares so much more about the user experience.)

But, short of a switch to Opera, how could I fix it on my end? Google wasn't any help, but I figured that this must be a reported Mozilla bug, so I turned to Bugzilla and found quite a lot. Here's the scoop. First, the code to respect the cache settings (slowing down Back/Forward) was apparently added in response to bug 101832. People quickly noticed the performance problem, and filed 112564. (This was back in late 2001.) There was a long debate, but in the end, a fix was checked in which allowed no-cache http (non-secure) sites to cache and get a fast Back/Forward. This didn't help no-store and must-revalidate sites, which were still just as slow as ever.

Then a few months later, bug 135289 changed this code around quite a bit. I'm still getting my head around the code involved in the two bugs, but I think this update didn't change the basic rules governing which pages get revalidated.

(Warning: geekage alert for next two paragraphs. Use this fix at your own risk, etc.)

Unfortunately, it looks like the only way to fix this is in the C++ code. For folks not afraid of building Firefox, the code lives in nsDocShell::ShouldDiscardLayoutState and controls the no-cache and no-store directives. In nsDocShell::ShouldDiscardLayoutState (currently lie 8224, but don't count on it), the final line is:

    return (noStore || (noCache && securityInfo));
Change that to
    return ((noStore || noCache) && securityInfo);
and Back/Forward will get instantly faster, while still preserving security for https. (If you don't care about that security issue and want pages to cache no matter what, just replace the whole function with return PR_FALSE; )

The must-validate setting is handled in a completely different place, in nsHttpChannel. However, for some reason, fixing nsDocShell also fixes Drupal pages which set only must-validate. I don't quite understand why yet. More study required. (End geekage.)

Any Mozilla folks are welcome to tell me why I shouldn't be doing this, or if there's a better way (especially if it's possible in a way that would work from an extension or preference). I'd also be interested in from Drupal or other CMS folks defending why so many CMSes destroy the user experience like this. But please first read the Opera article referenced above, so that you understand why I and so many other users have complained about it. I'm happy to share any comments I receive (let me know if you want your comments to be public or not).

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[ 19:32 Oct 20, 2007    More tech/web | permalink to this entry | comments ]

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