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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: Tim Smith <tim.smith@citrix.com>

Re-based and commit comment adjusted.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
This commit is contained in:
Tim Smith 2018-12-12 11:16:26 +00:00 committed by Anthony PERARD
parent bfd0d63660
commit c6025bd197
1 changed files with 9 additions and 5 deletions
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14
hw/block/dataplane/xen-block.c
View File

@ -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);
}