Urbit / Docs

u3: Land of nouns

The division between c3 and u3 is that you could theoretically imagine using c3 as just a generic C environment. Anything to do with nouns is in u3.

u3: a map of the system

There are two kinds of symbols in u3: regular and irregular. Regular symbols follow this pattern:

    prefix    purpose                      .h         .c
    -------------------------------------------------------
    u3a_      allocation                   i/n/a.h    n/a.c
    u3e_      persistence                  i/n/e.h    n/e.c
    u3h_      hashtables                   i/n/h.h    n/h.c
    u3i_      noun construction            i/n/i.h    n/i.c
    u3j_      jet control                  i/n/j.h    n/j.c
    u3m_      system management            i/n/m.h    n/m.c
    u3n_      nock computation             i/n/n.h    n/n.c
    u3r_      noun access (error returns)  i/n/r.h    n/r.c
    u3t_      profiling                    i/n/t.h    n/t.c
    u3v_      arvo                         i/n/v.h    n/v.c
    u3x_      noun access (error crashes)  i/n/x.h    n/x.c
    u3z_      memoization                  i/n/z.h    n/z.c
    u3k[a-g]  jets (transfer, C args)      i/j/k.h    j/[a-g]/*.c
    u3q[a-g]  jets (retain, C args)        i/j/q.h    j/[a-g]/*.c
    u3w[a-g]  jets (retain, nock core)     i/j/w.h    j/[a-g]/*.c

Irregular symbols always start with u3 and obey no other rules. They're defined in i/n/u.h. Finally, i/all.h includes all these headers (fast compilers, yay) and is all you need to program in u3.

u3: noun internals

A noun is a u3_noun - currently defined as a 32-bit c3_w.

If your u3_noun is less than (1 << 31), it's a direct atom. Every unsigned integer between 0 and 0x7fffffff inclusive is its own noun.

If bit 31 is set in a u3_noun, bit 30 is always set - this bit is reserved. Bit 29 is 1 if the noun is a cell, 0 if it's an atom. Bits 28 through 0 are a word pointer into the loom - see below. The structures are:

    typedef struct {
      c3_w mug_w;
      c3_w len_w;
      c3_w buf_w[0];    //  actually [len_w]
    } u3a_atom;

    typedef struct {
      c3_w    mug_w;
      u3_noun hed;
      u3_noun tel;
    } u3a_cell;

The only thing that should be mysterious here is mug_w, which is a 31-bit lazily computed nonzero short hash (FNV currently, soon Murmur3). If mug_w is 0, the hash is not yet computed. We also hijack this field for various hacks, such as saving the new address of a noun when copying over.

Also, the value 0xffffffff is u3_none, which is never a valid noun. Use the type u3_weak to express that a noun variable may be u3_none.

u3: reference counts

The only really essential thing you need to know about u3 is how to handle reference counts. Everything else, you can skip and just get to work.

u3 deals with reference-counted, immutable, acyclic nouns. Unfortunately, we are not Apple and can't build reference counting into your C compiler, so you need to count by hand.

Every allocated noun (or any allocation object, because our allocator is general-purpose) contains a counter which counts the number of references to it - typically variables with type u3_noun. When this counter goes to 0, the noun is freed.

To tell u3 that you've added a reference to a noun, call the function u3a_gain() or its shorthand u3k(). (For your convenience, this function returns its argument.) To tell u3 that you've destroyed a reference, call u3a_lose() or u3z().

(If you screw up by decrementing the counter too much, u3 will dump core in horrible ways. If you screw up by incrementing it too much, u3 will leak memory. To check for memory leaks, set the bug_o flag in u3e_boot() - eg, run vere with -g. Memory leaks are difficult to debug - the best way to handle leaks is just to revert to a version that didn't have them, and look over your code again.)

(You can gain or lose a direct atom. It does nothing.)

u3: reference protocols

THIS IS THE MOST CRITICAL SECTION IN THE u3 DOCUMENTATION.

The key question when calling a C function in a refcounted world is what the function will do to the noun refcounts - and, if the function returns a noun, what it does to the return.

There are two semantic patterns, transfer and retain. In transfer semantics, the caller “gives” a use count to the callee, which “gives back” any return. For instance, if I have

    {
      u3_noun foo = u3i_string("foobar");
      u3_noun bar;

      bar = u3f_futz(foo);
      [...]
      u3z(bar);
    }

Suppose u3f_futz() has transfer semantics. At [...], my code holds one reference to bar and zero references to foo - which has been freed, unless it's part of bar. My code now owns bar and gets to work with it until it's done, at which point a u3z() is required.

On the other hand, if u3f_futz() has retain semantics, we need to write

    {
      u3_noun foo = u3i_string("foobar");
      u3_noun bar;

      bar = u3f_futz(foo);
      [...]
      u3z(foo);
    }

because calling u3f_futz() does not release our ownership of foo, which we have to free ourselves.

But if we free bar, we are making a great mistake, because our reference to it is not in any way registered in the memory manager (which cannot track references in C variables, of course). It is normal and healthy to have these uncounted C references, but they must be treated with care.

The bottom line is that it's essential for the caller to know the refcount semantics of any function which takes or returns a noun. (In some unusual circumstances, different arguments or returns in one function may be handled differently.)

Broadly speaking, as a design question, retain semantics are more appropriate for functions which inspect or query nouns. For instance, u3h() (which takes the head of a noun) retains, so that we can traverse a noun tree without constantly incrementing and decrementing.

Transfer semantics are more appropriate for functions which make nouns, which is obviously what most functions do.

In general, though, in most places it's not worth thinking about what your function does. There is a convention for it, which depends on where it is, not what it does. Follow the convention.

u3: reference conventions

The u3 convention is that, unless otherwise specified, all functions have transfer semantics - with the exception of the prefixes: u3r, u3x, u3z, u3q and u3w. Also, within jet directories a through f (but not g), internal functions retain (for historical reasons).

If functions outside this set have retain semantics, they need to be commented, both in the .h and .c file, with RETAIN in all caps. Yes, it's this important.

u3: system architecture

If you just want to tinker with some existing code, it might be enough to understand the above. If not, it's probably worth taking the time to look at u3 as a whole.

u3 is designed to work as a persistent event processor. Logically, it computes a function of the form

    f(event, old state) -> (actions, new state)

Obviously almost any computing model - including, but not limited to, Urbit - can be defined in this form. To create the illusion of a computer that never loses state and never fails, we:

  • log every event externally before it goes into u3

  • keep a single reference to a permanent state noun.

  • can abort any event without damaging the permanent state.

  • snapshot the permanent state periodically, and/or prune logs.

u3: the road model

u3 uses a memory design which I'm sure someone has invented somewhere before, because it's not very clever, but I've never seen it anywhere in particular.

Every allocation starts with a solid block of memory, which u3 calls the loom. How do we allocate on the loom? You're probably familiar with the Unix heap-stack design, in which the stack grows downward and the heap (malloc arena) grows upward:

    0           brk                                          ffff
    |   heap     |                                    stack    |
    |------------#################################+++++++++++++|
    |                                             |            |
    0                                             sp         ffff

A road is a normal heap-stack system, except that the heap and stack can point in either direction. Therefore, inside a road, we can nest another road in the opposite direction.

When the opposite road completes, its heap is left on top of the opposite heap's stack. It's no more than the normal behavior of a stack machine for all subcomputations to push their results on the stack.

The performance tradeoff of “leaping” - reversing directions in the road - is that if the outer computation wants to preserve the results of the inner one, not just use them for temporary purposes, it has to copy them.

This is a trivial cost in some cases, a prohibitive cost in others. The upside, of course, is that all garbage accrued in the inner computation is discarded at zero cost.

The goal of the road system is the ability to layer memory models. If you are allocating on a road, you have no idea how deep within a nested road system you are - in other words, you have no idea exactly how durable your result may be. But free space is never fragmented within a road.

Roads do not reduce the generality or performance of a memory system, since even the most complex GC system can be nested within a road at no particular loss of performance - a road is just a block of memory.

Each road (u3a_road to be exact) uses four pointers: rut is the bottom of the arena, hat the top of the arena, mat the bottom of the stack, cap the top of the stack. (Bear in mind that the road “stack” is not actually used as the C function-call stack, though it probably should be.)

A “north” road has the stack high and the heap low:

    0           rut   hat                                    ffff
    |            |     |                                       |
    |~~~~~~~~~~~~-------##########################+++++++$~~~~~|
    |                                             |      |     |
    0                                            cap    mat  ffff

A “south” road is the other way around:

    0           mat   cap                                    ffff
    |            |     |                                       |
    |~~~~~~~~~~~~$++++++##########################--------~~~~~|
    |                                             |      |     |
    0                                            hat    rut  ffff

Legend: - is durable storage (heap); + is temporary storage (stack); ~ is deep storage (immutable); $ is the allocation frame # is free memory.

Pointer restrictions: pointers stored in + can point anywhere. Pointers in - can only point to - or ~; pointers in ~ only point to ~.

To “leap” is to create a new inner road in the ### free space. but in the reverse direction, so that when the inner road “falls” (terminates), its durable storage is left on the temporary storage of the outer road.

u3 keeps a global variable, u3_Road or its alias u3R, which points to the current road. (If we ever run threads in inner roads - see below - this will become a thread-local variable.) Relative to u3R, + memory is called junior memory; - memory is normal memory; ~ is senior memory.

u3: explaining the road model

But... why?

We're now ready to understand why the road system works so logically with the event and persistence model.

The key is that we don't update refcounts in senior memory. A pointer from an inner road to an outer road is not counted. Also, the outmost, or surface road, is the only part of the image that gets checkpointed.

So the surface road contains the entire durable state of u3. When we process an event, or perform any kind of complicated or interesting calculation, we process it in an inner road. If its results are saved, they need to be copied.

Since processing in an inner road does not touch surface memory, (a) we can leave the surface road in a read-only state and not mark its pages dirty; (b) we can abort an inner calculation without screwing up the surface; and (c) because inner results are copied onto the surface, the surface doesn't get fragmented.

All of (a), (b) and (c) are needed for checkpointing to be easy. It might be tractable otherwise, but easy is even better.

Moreover, while the surface is most definitely single-threaded, we could easily run multiple threads in multiple inner roads (as long as the threads don't have pointers into each others' memory, which they obviously shouldn't).

Moreover, in future, we'll experiment more with adding road control hints to the programmer's toolbox. Reference counting is expensive. We hypothesize that in many - if not most - cases, the programmer can identify procedural structures whose garbage should be discarded in one step by copying the results. Then, within the procedure, we can switch the allocator into sand mode, and stop tracking references at all.

u3: rules for C programming

There are two levels at which we program in C: (1) above the interpreter; (2) within the interpreter or jets. These have separate rules which need to be respected.

u3: rules above the interpreter

In its relations with Unix, Urbit follows a strict rule of “call me, I won't call you.” We do of course call Unix system calls, but only for the purpose of actually computing.

Above Urbit, you are in a normal C/Unix programming environment and can call anything in or out of Urbit. Note that when using u3, you're always on the surface road, which is not thread-safe by default. Generally speaking, u3 is designed to support event-oriented, single-threaded programming.

If you need threads which create nouns, you could use u3m_hate() and u3m_love() to run these threads in subroads. You'd need to make the global road pointer, u3R, a thread-local variable instead. This seems perfectly practical, but we haven't done it because we haven't needed to.

u3: rules within the interpreter

Within the interpreter, your code can run either in the surface road or in a deep road. You can test this by testing

    (&u3H->rod_u == u3R)

ie: does the pier's home road equal the current road pointer?

Normally in this context you assume you're obeying the rules of running on an inner road, ie, “deep memory.” Remember, however, that the interpreter can run on surface memory - but anything you can do deep, you can do on the surface. The converse is by no means the case.

In deep memory, think of yourself as if in a signal handler. Your execution context is extremely fragile and may be terminated without warning or cleanup at any time (for instance, by ctrl-c).

For instance, you can't call malloc (or C++ new) in your C code, because you don't have the right to modify data structures at the global level, and will leave them in an inconsistent state if your inner road gets terminated. (Instead, use our drop-in replacements, u3a_malloc(), u3a_free(), u3a_realloc().)

A good example is the different meaning of c3_assert() inside and outside the interpreter. At either layer, you can use regular assert(), which will just kill your process. On the surface, c3_assert() will just... kill your process.

In deep execution, c3_assert() will issue an exception that queues an error event, complete with trace stack, on the Arvo event queue. Let's see how this happens.

u3: exceptions

You produce an exception with

    /* u3m_bail(): bail out.  Does not return.
    **
    **  Bail motes:
    **
    **    %exit               ::  semantic failure
    **    %evil               ::  bad crypto
    **    %intr               ::  interrupt
    **    %fail               ::  execution failure
    **    %foul               ::  assert failure
    **    %need               ::  network block
    **    %meme               ::  out of memory
    **    %time               ::  timed out
    **    %oops               ::  assertion failure
    */
      c3_i
      u3m_bail(c3_m how_m);

Broadly speaking, there are two classes of exception: internal and external. An external exception begins in a Unix signal handler. An internal exception begins with a call to longjmp() on the main thread.

There are also two kinds of exception: mild and severe. An external exception is always severe. An internal exception is normally mild, but some (like c3__oops, generated by c3_assert()) are severe.

Either way, exceptions come with a stack trace. The u3 nock interpreter is instrumented to retain stack trace hints and produce them as a printable (list tank).

Mild exceptions are caught by the first virtualization layer and returned to the caller, following the behavior of the Nock virtualizer ++mock (in hoon.hoon)

Severe exceptions, or mild exceptions at the surface, terminate the entire execution stack at any depth and send the cumulative trace back to the u3 caller.

For instance, vere uses this trace to construct a %crud event, which conveys our trace back toward the Arvo context where it crashed. This lets any UI component anywhere, even on a remote node, render the stacktrace as a consequence of the user's action - even if its its direct cause was (for instance) a Unix SIGINT or SIGALRM.

u3: C structures on the loom

Normally, all data on the loom is nouns. Sometimes we break this rule just a little, though - eg, in the u3h hashtables.

To point to non-noun C structs on the loom, we use a u3_post, which is just a loom word offset. A macro lets us declare this as if it was a pointer:

    typedef c3_w       u3_post;
    #define u3p(type)  u3_post

Some may regard this as clever, others as pointless. Anyway, use u3to() and u3of() to convert to and from pointers.

When using C structs on the loom - generally a bad idea - make sure anything which could be on the surface road is structurally portable, eg, won't change size when the pointer size changes. (Note also: we consider little-endian, rightly or wrongly, to have won the endian wars.)