How to efficiently draw framebuffer content?

Question

I need to display RAM-based framebuffer for a virtual GPU device that doesn't have real display connected to it. What I have is mmap'ed chunk of memory after DRM_IOCTL_MODE_MAP_DUMB in RGB32 format. Currently I'm using MIT-SHM shared pixmap created via XShmCreatePixmap() like this: ``` shminfo.shmid = shmget(IPC_PRIVATE, bytes, IPC_CREAT|0777); shminfo.readOnly = False; shminfo.shmaddr = shmat(shminfo.shmid, 0, 0); shmctl(shminfo.shmid, IPC_RMID, 0); XShmAttach(dpy, &shminfo); XShmCreatePixmap(dpy, window, shminfo.shmaddr, &shminfo, width, height, 24); ``` and then simply ``` while (1) { struct timespec ts = {0, 999999999L / 30}; nanosleep(&ts, NULL); memcpy(shminfo.shmaddr, mem, bytes); XCopyArea(dpy, pixmap, window, gc, 0, 0, width, height, 0, 0); XFlush(dpy); } ``` So it loops 30 times per second, doing memcpy followed XCopyArea. The problem is that it uses a lot of CPU: 50% on a powerful machine. Is there any better way? I could think of two possible improvements: Get rid of memcp…

Accepted Answer

For X11 use `XShmCreateImage`, write to `XImage.data` and make visible with `XShmPutImage` making sure to pass `False` for `send_event` parameter. You may also want to disable exposure events for the current GC; setting `PointerMotionHintMask` can also help.

SDL1 does most of the above but will use a shadow surface if there is a mismatch between user and display format and may perform unexpected color conversion. SDL2 tries to use hardware acceleration and may perform unexpected scaling and/or filtering. Make sure you're getting what you ask for to avoid hidden ops.

%50 cpu usage sounds like a lot for this blit at 30fps, I'd rewrite the sleep function as follows just in case.

```
do
    errno = 0;
while ( nanosleep(&ts, &ts) && errno == EINTR );
```

How to efficiently draw framebuffer content?

Solution

Problem