From 916237fd78ff690b56bf849df1d9b628613f70fd Mon Sep 17 00:00:00 2001
From: Jiaming Yuan <jm.yuan@outlook.com>
Date: Sat, 22 Apr 2023 03:48:58 +0800
Subject: [PATCH] Code comment.

---
 src/data/iterative_dmatrix.cc | 4 ++--
 src/data/iterative_dmatrix.cu | 3 ++-
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/src/data/iterative_dmatrix.cc b/src/data/iterative_dmatrix.cc
index 8b4ae524ef0f..799b1848bd15 100644
--- a/src/data/iterative_dmatrix.cc
+++ b/src/data/iterative_dmatrix.cc
@@ -301,8 +301,8 @@ BatchSet<GHistIndexMatrix> IterativeDMatrix::GetGradientIndex(Context const* ctx
     } else if (fmat_ctx_.IsCPU()) {
       ghist_ = std::make_shared<GHistIndexMatrix>(&fmat_ctx_, Info(), *ellpack_, param);
     } else {
-      // Can happen when QDM is initialized on GPU, but a CPU version is queried by to_csr
-      // or the predictor.
+      // Can happen when QDM is initialized on GPU, but a CPU version is queried by a different QDM
+      // for cut reference.
       auto cpu_ctx = ctx->MakeCPU();
       ghist_ = std::make_shared<GHistIndexMatrix>(&cpu_ctx, Info(), *ellpack_, param);
     }
diff --git a/src/data/iterative_dmatrix.cu b/src/data/iterative_dmatrix.cu
index c96c1cc52104..f23bbd5a14f0 100644
--- a/src/data/iterative_dmatrix.cu
+++ b/src/data/iterative_dmatrix.cu
@@ -192,7 +192,8 @@ BatchSet<EllpackPage> IterativeDMatrix::GetEllpackBatches(Context const* ctx,
       *ellpack_->Impl() =
           EllpackPageImpl(&fmat_ctx_, *this->ghist_, this->Info().feature_types.ConstDeviceSpan());
     } else {
-      // Can happen when QDM is initialized on CPU, but a GPU version is queried by the predictor.
+      // Can happen when QDM is initialized on CPU, but a GPU version is queried by a different QDM
+      // for cut reference.
       auto cuda_ctx = ctx->MakeCUDA();
       this->Info().feature_types.SetDevice(cuda_ctx.gpu_id);
       *ellpack_->Impl() =