[EM] Make device cache more conservative. (#11547)

Author: trivialfis

src/data/batch_utils.cc

@@ -84,7 +84,7 @@ void CheckParam(BatchParam const& init, BatchParam const& param) {
     cache_host_ratio = 0.0;
   } else {
     // The number of bytes that must be in the host memory.
-    auto h_cache_nbytes = n_cache_bytes - d_cache_nbytes;
+    auto h_cache_nbytes = n_cache_bytes - d_cache_nbytes * 0.85;
     cache_host_ratio = static_cast<double>(h_cache_nbytes) / static_cast<double>(n_cache_bytes);
     if (lc > 0) {
       // 0 < lc < 10, C2C is available, but with reduced link count.

src/data/ellpack_page_source.cu

@@ -181,7 +181,7 @@ class EllpackHostCacheStreamImpl {
       new_impl->CopyInfo(old_impl);
 
       // Split the cache into host cache, compressed host cache, and the device cache. We
-      // use the decompression engin only for sparse data.
+      // use the decompression engine only for sparse data.
       auto n_bytes = get_host_nbytes(old_impl);
       CHECK_LE(n_bytes, old_impl->gidx_buffer.size_bytes());
       std::size_t n_h_bytes = n_bytes, n_comp_bytes = 0;