posix_data_source_impl: adaptive prefetch size

Signed-off-by: Yingxin <[email protected]>

include/seastar/net/posix-stack.hh

@@ -105,8 +105,10 @@ class posix_data_source_impl final : public data_source_impl {
     lw_shared_ptr<pollable_fd> _fd;
     temporary_buffer<char> _buf;
     size_t _buf_size;
+    static constexpr size_t min_buf_size = 1 << 9;
+    static constexpr size_t max_buf_size = 1 << 19;
 public:
-    explicit posix_data_source_impl(lw_shared_ptr<pollable_fd> fd, size_t buf_size = 8192)
+    explicit posix_data_source_impl(lw_shared_ptr<pollable_fd> fd, size_t buf_size = 1 << 13)
         : _fd(std::move(fd)), _buf(buf_size), _buf_size(buf_size) {}
     future<temporary_buffer<char>> get() override;
     future<> close() override;

src/net/posix-stack.cc

@@ -316,6 +316,11 @@ posix_data_source_impl::get() {
     return _fd->read_some(_buf.get_write(), _buf_size).then([this] (size_t size) {
         _buf.trim(size);
         auto ret = std::move(_buf);
+        if (_buf_size == size) {
+          _buf_size = std::min(max_buf_size, _buf_size << 2);
+        } else if (size < (_buf_size >> 2)) {
+          _buf_size = std::max(min_buf_size, _buf_size >> 2);
+        }
         _buf = temporary_buffer<char>(_buf_size);
         return make_ready_future<temporary_buffer<char>>(std::move(ret));
     });