Sunday, 3 November 2013

It worked again!

Just had my first success at launching a cooperative multi-core programme on the epiphany using my own loader.
    ... lots of spew ...
    Loading workgroup
     writing 'e-test-workgroup-b.elf' len  8208 to core  0, 1  0xb6f11000 <- 0x1d618
     writing 'e-test-workgroup-b.elf' len  8208 to core  1, 1  0xb6e1a000 <- 0x1d618
     writing 'e-test-workgroup-b.elf' len  8208 to core  2, 1  0xb6e08000 <- 0x1d618
     writing 'e-test-workgroup-b.elf' len  8208 to core  3, 1  0xb6df6000 <- 0x1d618
     writing 'e-test-workgroup-a.elf' len  2808 to core  0, 0  0xb6f19000 <- 0x154c8
     writing 'e-test-workgroup-a.elf' len  2808 to core  1, 0  0xb6e23000 <- 0x154c8
     writing 'e-test-workgroup-a.elf' len  2808 to core  2, 0  0xb6e11000 <- 0x154c8
     writing 'e-test-workgroup-a.elf' len  2808 to core  3, 0  0xb6dff000 <- 0x154c8
    find symbol _shared
     section base = 8e000000 val=00000000
    Global shared = 0xb4df5000
    Reset cores
    Start cores: 0,0 0,1 1,0 1,1 2,0 2,1 3,0 3,1
    Waiting for results
    result 0 = 000013ab
    result 1 = 000130ab
    result 2 = 001300ab
    result 3 = 013000ab

I have two columns each with different programmes. The two cores in each row communicates individually on their problem and writes the result to shared memory. I came up with a queing primitive an 'eport' which handles arbitration on a 1:1 basis.

Although the per-core e_workgroup_config is in the elf file, it is uninitialised, so I had to set that up during the loading phase ... which took me an hour or so of faffing about to realise ...

The Code

So the whole point of this exercise is to write non-hetereogeneous multi-core programmes without having to hard-code addresses or manually extract symbols from elf files. The code below demonstrates that i've managed this to about as good a level as one can expect without having to customise any of the toolchain.

Program A defines the global shared resource (even though in this case it doesn't use it), a local eport endpoint, and a remote queue address (bbuffer).

struct interface shared __attribute__((section(".bss.shared")));

extern struct eport bport __attribute__((weak));
extern int bbuffer[4] __attribute__((weak));

struct eport aport = EPORT_INIT(4);

int main(void) {
        int lid = e_group_config.core_row;
        int *bbufferp;

        bbufferp = e_get_global_address(lid, 1, bbuffer);

        eport_setup(&aport, e_get_global_address(lid, 1, &bport), 4);

        for (int i=0;i<20;i++) {
                int wid = eport_reserve(&aport);

                bbufferp[wid] = i == 19 ? -1 : (i + (1<<((4*lid)+8)));

It feeds out numbers to add one at a time to the ecore in the same row but in column 1. The 'eport' handles arbitration and blocking and so no 'critical section' arises at either end by passing ownership from the queue slot from one to the other (well it will when i fix a bug i just spotted) in an asynchronous manner; which is critical for performance.

Program B takes the values as they arrive adding them up - until a sentinal arrives. It then saves the result and pings the host.

extern struct interface shared __attribute__((weak));
extern struct eport aport __attribute__((weak));

struct eport bport = EPORT_INIT(4);
int bbuffer[4] __attribute__((section(".data.bank1")));

int main(void) {
        int lid = e_group_config.core_row;
        unsigned int qid;
        int val;
        int sum = 0;

        eport_setup(&bport, e_get_global_address(lid, 0, &aport), 4);

        do {
                qid = eport_wait(&bport);

                val = bbuffer[qid];

                if (val != -1)
                        sum += val;

        } while (val != -1);

        shared.sum[lid] = sum;
        shared.c[lid] = 1;

The only 'pain' is that each endpoint of the eport needs to be pointed to the other end explicitly - it can't be handled in the linker. Oh and symbols outside of the sub-workgroup must be accessed using weak references, which can be a bit easy to mess up (I think i can use void *, which helps protect against errors). These ecore programmes must be linked with '-r' to create relocatable elf files but otherwise do not need a linker script.

The host code is fairly straightforward too although I haven't yet got anything tidy for the return communications (actually eport would work too but you'd need one pair for each result core).

        struct interface *shared;

        wg = ez_open(0, 0, 4, 2);

        ez_bind(wg, "e-test-workgroup-a.elf", 0, 0, 4, 1);
        ez_bind(wg, "e-test-workgroup-b.elf", 0, 1, 4, 1);

        shared = ez_addr(wg, 0, 0, "_shared");

        for (int i=0;i<4;i++)
                shared->c[i] = 0;

        for (int r=0;r<4;r++) {
                while (shared->c[r] == 0)
                printf("result %d = %08x\n", r, shared->sum[r]);

Still a few bugs and things to finish off but it's a solid foundation.

Unfortunately ... it's nowhere near as trivial as loading a demand-paged elf file. The code has to do some of the linking and a few other bits and pieces - it's about 1 000 lines at the moment.

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