On 12/17/19 3:39 PM, Jens Axboe wrote:
If RWF_UNCACHED is set for io_uring (or preadv2(2)), we'll use private pages for the buffered reads. These pages will never be inserted into the page cache, and they are simply droped when we have done the copy at the end of IO. If pages in the read range are already in the page cache, then use those for just copying the data instead of starting IO on private pages. A previous solution used the page cache even for non-cached ranges, but the cost of doing so was too high. Removing nodes at the end is expensive, even with LRU bypass. On top of that, repeatedly instantiating new xarray nodes is very costly, as it needs to memset 576 bytes of data, and freeing said nodes involve an RCU call per node as well. All that adds up, making uncached somewhat slower than O_DIRECT. With the current*solition*, we're basically at O_DIRECT levels of
Maybe it is 'solution' here. Thanks, Guoqing
