Re: [PATCH] drm/i915: Use hash tables for the command parser

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 05/10/2014 10:10 PM, bradley.d.volkin@intel.com wrote:
> From: Brad Volkin <bradley.d.volkin@intel.com>
>
> For clients that submit large batch buffers the command parser has
> a substantial impact on performance. On my HSW ULT system performance
> drops as much as ~20% on some tests. Most of the time is spent in the
> command lookup code. Converting that from the current naive search to
> a hash table lookup reduces the performance drop to ~10%.
>
> The choice of value for I915_CMD_HASH_ORDER allows all commands
> currently used in the parser tables to hash to their own bucket (except
> for one collision on the render ring). The tradeoff is that it wastes
> memory. Because the opcodes for the commands in the tables are not
> particularly well distributed, reducing the order still leaves many
> buckets empty. The increased collisions don't seem to have a huge
> impact on the performance gain, but for now anyhow, the parser trades
> memory for performance.
>
> NB: Ville noticed that the error paths through the ring init code
> will leak memory. I've not addressed that here. We can do a follow
> up pass to handle all of the leaks.
>
> v2: improved comment describing selection of hash key mask (Damien)
> replace a BUG_ON() with an error return (Tvrtko, Ville)
> commit message improvements
>
> Signed-off-by: Brad Volkin <bradley.d.volkin@intel.com>

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

Regards,

Tvrtko