[PATCH 0/9] Visible VRAM Management Improvements

John Brooks john at fastquake.com
Fri Jun 23 17:39:31 UTC 2017

This patch series is intended to improve performance when limited CPU-visible
VRAM is under pressure.

Moving BOs into visible VRAM is essentially a housekeeping task. It's faster to
access them in VRAM than GTT, but it isn't a hard requirement for them to be in
VRAM. As such, it is unnecessary to spend valuable time blocking on this in the
page fault handler or during command submission. Doing so translates directly
into a longer frame time (ergo stalls and stuttering).

The problem worsens when attempting to move BOs into visible VRAM when it is
full. This takes much longer than a simple move because other BOs have to be
evicted, which involves finding and then moving potentially hundreds of other
BOs, which is very time consuming. In the case of limited visible VRAM, it's
important to do this sometime to keep the contents of visible VRAM fresh, but
it does not need to be a blocking operation. If visible VRAM is full, the BO
can be read from GTT in the meantime and the BO can be moved to VRAM later.

Thus, I have made it so that neither the command submission code nor page fault
handler spends time evicting BOs from visible VRAM, and instead this is
deferred to a workqueue function that's queued when CS requests BOs flagged

Speaking of CPU_ACCESS_REQUIRED, I've changed the handling of that flag so that
the kernel driver can clear it later even if it was set by userspace. This is
because the userspace graphics library can't know whether the application
really needs it to be CPU_ACCESS_REQUIRED forever. The kernel driver can't know
either, but it does know when page faults occur, and if a BO doesn't appear to
have any page faults when it's moved somewhere inaccessible, the flag can be
removed and it doesn't have to take up space in CPU-visible memory anymore.
This change was based on IRC discussions with Michel.

Patch 7 fixes a problem with BO moverate throttling that causes visible VRAM
moves to not be throttled if total VRAM isn't full enough.

I've also added a vis_vramlimit module parameter for debugging purposes. It's
similar to the vramlimit parameter except it limits only visible VRAM.

I have tested this patch set with the two games I know to be affected by
visible VRAM pressure: DiRT Rally and Dying Light. It practically eliminates
eviction-related stuttering in DiRT Rally as well as very low performance if
visible VRAM is limited to 64MB. It also fixes severely low framerates that
occurred in some areas of Dying Light. All my testing was done with an R9 290
with 4GB of visible VRAM with an Intel i7 4790.

John Brooks (Frogging101)

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