Commit c6025bd1 authored by Tim Smith's avatar Tim Smith Committed by Anthony PERARD
Browse files

xen-block: avoid repeated memory allocation 



The xen-block dataplane currently allocates memory to hold the data for
each request as that request is used, and frees it afterwards. Because
it requires page-aligned blocks, this interacts poorly with non-page-
aligned allocations and balloons the heap.

Instead, allocate the maximum possible buffer size required for the
protocol, which is BLKIF_MAX_SEGMENTS_PER_REQUEST (currently 11) pages
when the request structure is created, and keep that buffer until it is
destroyed. Since the requests are re-used via a free list, this should
actually improve memory usage.

Signed-off-by: default avatarTim Smith <tim.smith@citrix.com>

Re-based and commit comment adjusted.

Signed-off-by: default avatarPaul Durrant <paul.durrant@citrix.com>
Acked-by: default avatarAnthony PERARD <anthony.perard@citrix.com>
Signed-off-by: default avatarAnthony PERARD <anthony.perard@citrix.com>
parent bfd0d636

hw/block/dataplane/xen-block.c

+9 −5
Original line number Diff line number Diff line
@@ -70,7 +70,6 @@ static void reset_request(XenBlockRequest *request) 
    memset(&request->req, 0, sizeof(request->req));

    request->status = 0;

    request->start = 0;

    request->buf = NULL;

    request->size = 0;

    request->presync = 0;
@@ -95,6 +94,14 @@ static XenBlockRequest *xen_block_start_request(XenBlockDataPlane *dataplane) 
        /* allocate new struct */

        request = g_malloc0(sizeof(*request));

        request->dataplane = dataplane;

        /*

         * We cannot need more pages per requests than this, and since we

         * re-use requests, allocate the memory once here. It will be freed

         * xen_block_dataplane_destroy() when the request list is freed.

         */

        request->buf = qemu_memalign(XC_PAGE_SIZE,

                                     BLKIF_MAX_SEGMENTS_PER_REQUEST *

                                     XC_PAGE_SIZE);

        dataplane->requests_total++;

        qemu_iovec_init(&request->v, 1);

    } else {
@@ -272,14 +279,12 @@ static void xen_block_complete_aio(void *opaque, int ret) 
        if (ret == 0) {

            xen_block_copy_request(request);

        }

        qemu_vfree(request->buf);

        break;

    case BLKIF_OP_WRITE:

    case BLKIF_OP_FLUSH_DISKCACHE:

        if (!request->req.nr_segments) {

            break;

        }

        qemu_vfree(request->buf);

        break;

    default:

        break;
@@ -360,12 +365,10 @@ static int xen_block_do_aio(XenBlockRequest *request) 
{

    XenBlockDataPlane *dataplane = request->dataplane;



    request->buf = qemu_memalign(XC_PAGE_SIZE, request->size);

    if (request->req.nr_segments &&

        (request->req.operation == BLKIF_OP_WRITE ||

         request->req.operation == BLKIF_OP_FLUSH_DISKCACHE) &&

        xen_block_copy_request(request)) {

        qemu_vfree(request->buf);

        goto err;

    }
@@ -665,6 +668,7 @@ void xen_block_dataplane_destroy(XenBlockDataPlane *dataplane) 
        request = QLIST_FIRST(&dataplane->freelist);

        QLIST_REMOVE(request, list);

        qemu_iovec_destroy(&request->v);

        qemu_vfree(request->buf);

        g_free(request);

    }