[Barrelfish-users] Fwd: How to reclaim the memory allocated by frame_alloc?

[Baumann Andrew]

Hi,

This is a very inefficient way to send a message. Sending the cap is an expensive operation. If the messages you are sending are relatively small (e.g. a few hundred bytes), you can send them as a byte array (e.g. uint8 buf[len] in a flounder spec). If the messages are large, you should probably look into using the bulk transport library, which uses shared memory under the covers, but is more efficient about reusing the same memory and not transferring capabilities with each message.

To answer your direct question, it looks like what you are doing is correct, but I doubt that the system correctly reclaims all the memory, and I know that revoke is broken across cores in the current tree. You may also find it is easier to unmap and delete the cap on both cores, rather than revoking it on the sender side.

Andrew

From: Shi Jinghao (史经浩） [mailto:jhshi at cs.hku.hk]
Sent: Tuesday, 27 March, 2012 6:28
To: barrelfish-users at lists.inf.ethz.ch
Subject: [Barrelfish-users] Fwd: How to reclaim the memory allocated by frame_alloc?

Just realize that I sent the mail using gmail, which seems be filtered by this maillist. Please ignore this
if you've seen it.

Regards,
Jinghao
---------- Forwarded message ----------
From: Shi Jinghao <jhshi89 at gmail.com<mailto:jhshi89 at gmail.com>>
Date: Tue, Mar 27, 2012 at 9:23 PM
Subject: How to reclaim the memory allocated by frame_alloc?
To: barrelfish-users at lists.inf.ethz.ch<mailto:barrelfish-users at lists.inf.ethz.ch>


Hi,

I need to send message between cores in barrelfish (X86_32). And somehow I decide to send the capability
of a frame that contains the message, since the message will have variable size. And I was wondering what's
the proper way to reclaim the message frame when it's useless.

I create a message frame like this (details like error checking are omitted, also for simplicity, assume message
size is fixed (MSG_SIZE)):

    // allocate a frame capability
    struct capref* msgcap = (struct capref*) malloc (sizeof(struct capref));
    frame_alloc(msgcap, MSG_SIZE, NULL);

    // map to my address space so I can fill the message body
    struct my_message* msg;
    vspace_map_one_frame_attr((void**)&msg, MSG_SIZE, *msgcap, VREGION_FLAGS_READ_WRITE, NULL, NULL);

Then I send the frame cap to another process (on a different core).

On receiver side, I get the message like this:

    struct my_message* msg;
    vspace_map_one_frame_attr(&msg, MSG_SIZE, msgcap, VREGION_FLAGS_READ_WRITE, NULL, NULL);

However, how am I supposed to reclaim the message? Based on my understanding, my current try is:

    // on sender side
    cap_revoke(*msgcap);
    cap_destroy(*msgcap);
    free(msgcap);  // msgcap is allocated by malloc
    vspace_unmap(msg);

    // on receiver side
    vspace_unmap(msg);

But I don't know if this is the right way to do it. (Though no explicit problem occurs until now). Any suggestions on this?

PS: on receiver side, vspace_map_on_frame_attr won't allocate any new memory, right? So after this, sender and receiver
will share the same physical memory. Please clarify if my understanding is right.

Thanks for your reading.

Regards,
Jinghao

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