Optimize execution time of the memdump tool

In GalaxyNote3 device, the execution time of memdump takes 5.3s.
Among overall time, GetPagesForMemoryMap function took more than 4s.
This patch removes unnecessary divisions and lseek operations for the hot loop in GetPagesForMemoryMap.
It reduced the execution time from 5.3s to 4.7s on the GalaxyNote3.

BUG=none

Review URL: https://codereview.chromium.org/177003017

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253792 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 9 insertions, 2 deletions

diff --git a/tools/android/memdump/memdump.cc b/tools/android/memdump/memdump.cc
index 50a5914..20684f6 100644
--- a/tools/android/memdump/memdump.cc
+++ b/tools/android/memdump/memdump.cc
@@ -200,15 +200,22 @@ bool GetPagesForMemoryMap(int pagemap_fd,
                           const MemoryMap& memory_map,
                           std::vector<PageInfo>* committed_pages,
                           BitSet* committed_pages_bits) {
+  const off64_t offset = memory_map.start_address / kPageSize;
+  if (lseek64(pagemap_fd, offset * sizeof(PageMapEntry), SEEK_SET) < 0) {
+    PLOG(ERROR) << "lseek";
+    return false;
+  }
   for (uint addr = memory_map.start_address, page_index = 0;
        addr < memory_map.end_address;
        addr += kPageSize, ++page_index) {
     DCHECK_EQ(0, addr % kPageSize);
     PageMapEntry page_map_entry = {};
     COMPILE_ASSERT(sizeof(PageMapEntry) == sizeof(uint64), unexpected_size);
-    const off64_t offset = addr / kPageSize;
-    if (!ReadFromFileAtOffset(pagemap_fd, offset, &page_map_entry))
+    ssize_t bytes = read(pagemap_fd, &page_map_entry, sizeof(page_map_entry));
+    if (bytes != sizeof(PageMapEntry) && bytes != 0) {
+      PLOG(ERROR) << "read";
       return false;
+    }
     if (page_map_entry.present) {  // Ignore non-committed pages.
       if (page_map_entry.page_frame_number == 0)
         continue;