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Code Issues Releases Activity

lavd/xcbgrab: do not try to create refcounted packets.

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The framework will allocate a buffer and copy the data to it,
that takes time. But it avoids constently creating and
destroyng the shared memory segment, and that saves more time.

On my setup,
from ~200 to ~300 FPS at full screen (1920×1200),
from ~1400 to ~3300 at smaller size (640×480),
similar to legacy x11grab and confirmed by others.

Plus, shared memory segments are a scarce resource,
allocating potentially many is a bad idea.

Note: if the application were to drop all references to the
buffer before the next call to av_read_frame(), then passing
the shared memory segment as a refcounted buffer would be
even more efficient, but it is hard to guarantee, and it does
not happen with the ffmpeg command-line tool. Using a small
number of preallocated buffers and resorting to a copy when
the pool is exhausted would be a solution to get the better
of both worlds.
This commit is contained in:
Nicolas George 2016-10-23 13:57:41 +02:00
parent 4abe1ff08f
commit 0bd1be65e8
1 changed files with 35 additions and 30 deletions
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65
libavdevice/xcbgrab.c
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@ -49,6 +49,8 @@
typedef struct XCBGrabContext {
const AVClass *class;
uint8_t *buffer;
xcb_connection_t *conn;
xcb_screen_t *screen;
xcb_window_t window;
@ -219,22 +221,16 @@ static int check_shm(xcb_connection_t *conn)
return 0;
}
static void dealloc_shm(void *unused, uint8_t *data)
{
shmdt(data);
}
static int xcbgrab_frame_shm(AVFormatContext *s, AVPacket *pkt)
static int allocate_shm(AVFormatContext *s)
{
XCBGrabContext *c = s->priv_data;
xcb_shm_get_image_cookie_t iq;
xcb_shm_get_image_reply_t *img;
xcb_drawable_t drawable = c->screen->root;
uint8_t *data;
int size = c->frame_size + AV_INPUT_BUFFER_PADDING_SIZE;
int id = shmget(IPC_PRIVATE, size, IPC_CREAT | 0777);
xcb_generic_error_t *e = NULL;
uint8_t *data;
int id;
if (c->buffer)
return 0;
id = shmget(IPC_PRIVATE, size, IPC_CREAT | 0777);
if (id == -1) {
char errbuf[1024];
int err = AVERROR(errno);
@ -243,15 +239,31 @@ static int xcbgrab_frame_shm(AVFormatContext *s, AVPacket *pkt)
size, errbuf);
return err;
}
xcb_shm_attach(c->conn, c->segment, id, 0);
data = shmat(id, NULL, 0);
shmctl(id, IPC_RMID, 0);
if ((intptr_t)data == -1 || !data)
return AVERROR(errno);
c->buffer = data;
return 0;
}
static int xcbgrab_frame_shm(AVFormatContext *s, AVPacket *pkt)
{
XCBGrabContext *c = s->priv_data;
xcb_shm_get_image_cookie_t iq;
xcb_shm_get_image_reply_t *img;
xcb_drawable_t drawable = c->screen->root;
xcb_generic_error_t *e = NULL;
int ret;
ret = allocate_shm(s);
if (ret < 0)
return ret;
iq = xcb_shm_get_image(c->conn, drawable,
c->x, c->y, c->width, c->height, ~0,
XCB_IMAGE_FORMAT_Z_PIXMAP, c->segment, 0);
xcb_shm_detach(c->conn, c->segment);
img = xcb_shm_get_image_reply(c->conn, iq, &e);
xcb_flush(c->conn);
@ -264,25 +276,12 @@ static int xcbgrab_frame_shm(AVFormatContext *s, AVPacket *pkt)
e->response_type, e->error_code,
e->sequence, e->resource_id, e->minor_code, e->major_code);
shmctl(id, IPC_RMID, 0);
return AVERROR(EACCES);
}
free(img);
data = shmat(id, NULL, 0);
shmctl(id, IPC_RMID, 0);
if ((intptr_t)data == -1)
return AVERROR(errno);
pkt->buf = av_buffer_create(data, size, dealloc_shm, NULL, 0);
if (!pkt->buf) {
shmdt(data);
return AVERROR(ENOMEM);
}
pkt->data = pkt->buf->data;
pkt->data = c->buffer;
pkt->size = c->frame_size;
return 0;
@ -436,6 +435,12 @@ static av_cold int xcbgrab_read_close(AVFormatContext *s)
{
XCBGrabContext *ctx = s->priv_data;
#if CONFIG_LIBXCB_SHM
if (ctx->buffer) {
shmdt(ctx->buffer);
}
#endif
xcb_disconnect(ctx->conn);
return 0;