More about my issues with DTrace's language

June 29, 2012

In his comment on my entry about why we haven't taken to DTrace, Brendan Gregg wrote in part:

It's been mentioned a few times, but I suspect it would be possible to create a higher level language ("D++") that speaks to libdtrace. An advantage of D being low level is that the user is conscious of how the system is actually getting traced, in the same way that C is low level. [...]

I don't think that this is the case. In fact I think it works the other way around; I doubt very much that people can go from D's limitations to any real understanding how the system is traced, but if you know how DTrace is implemented you can see the bones of this implementation underneath some of D's oddities.

To start with, I will agree that making some things clear is useful and even important. For example, I think that access to kernel data and variables should look different than access to user level data (and in a way that makes access to user level data look more expensive). What I object to is things that D makes pointlessly difficult, things where it doesn't support the obvious simple way of doing whatever and forces you to be indirect. The shining example of this is conditionals. D does not have any form of an if statement that you can use in the action that fires for a particular probe; however, probes themselves can be conditional, based on an expression. So you're left to fake an if by writing your entire probe action twice (and yes, I've done this in DTrace code).

The story I remember hearing about why this limitation exists is that the DTrace implementation doesn't want to be dynamically allocating output buffer space as a probe action executes; it wants to allocate the space once, before the probe's action starts. Well, fine, but if this is the reason you can deal with it in if-using D code by allocating the maximum amount of space the code might need if it followed the most pessimistic, space consuming path through the conditionals. Alternately, you could transform ifs by automatically creating multiple specific probes with probe conditions. Forcing DTrace users to duplicate their code in order to do this by hand is perverse, or at least an excessive focus on being literal about how DTrace's internals behave.

(You can argue that it saves users from themselves under some circumstances, for example if a rare condition requires a bunch more buffer space than the common ones. But this is an optimization and generally a premature one.)

Now, this story is clearly not the complete explanation given that DTrace has plenty of things that certainly look like they create variable sized output (including an outright ternary ?: operator). This pretty much illustrates my point, in that running into this D constraint hasn't made me any better informed than before about how the system is actually traced. It's still a black box, it's just a more frustrating black box.

Written on 29 June 2012.
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Last modified: Fri Jun 29 01:52:20 2012
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