[Qemu-devel] [PATCH RFC] mem-prealloc: Reduce large guest start-up and migration time.

[Jitendra Kolhe]

Using "-mem-prealloc" option for a very large guest leads to huge guest
start-up and migration time. This is because with "-mem-prealloc" option
qemu tries to map every guest page (create address translations), and
make sure the pages are available during runtime. virsh/libvirt by
default, seems to use "-mem-prealloc" option in case the guest is
configured to use huge pages. The patch tries to map all guest pages
simultaneously by spawning multiple threads. Given the problem is more
prominent for large guests, the patch limits the changes to the guests
of at-least 64GB of memory size. Currently limiting the change to QEMU
library functions on POSIX compliant host only, as we are not sure if
the problem exists on win32. Below are some stats with "-mem-prealloc"
option for guest configured to use huge pages.

------------------------------------------------------------------------
Idle Guest      | Start-up time | Migration time
------------------------------------------------------------------------
Guest stats with 2M HugePage usage - single threaded (existing code)
------------------------------------------------------------------------
64 Core - 4TB   | 54m11.796s    | 75m43.843s
64 Core - 1TB   | 8m56.576s     | 14m29.049s
64 Core - 256GB | 2m11.245s     | 3m26.598s
------------------------------------------------------------------------
Guest stats with 2M HugePage usage - map guest pages using 8 threads
------------------------------------------------------------------------
64 Core - 4TB   | 5m1.027s      | 34m10.565s
64 Core - 1TB   | 1m10.366s     | 8m28.188s
64 Core - 256GB | 0m19.040s     | 2m10.148s
-----------------------------------------------------------------------
Guest stats with 2M HugePage usage - map guest pages using 16 threads
-----------------------------------------------------------------------
64 Core - 4TB   | 1m58.970s     | 31m43.400s
64 Core - 1TB   | 0m39.885s     | 7m55.289s
64 Core - 256GB | 0m11.960s     | 2m0.135s
-----------------------------------------------------------------------

Signed-off-by: Jitendra Kolhe <address@hidden>
---
 util/oslib-posix.c | 64 +++++++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 61 insertions(+), 3 deletions(-)

diff --git a/util/oslib-posix.c b/util/oslib-posix.c
index f631464..a8bd7c2 100644
--- a/util/oslib-posix.c
+++ b/util/oslib-posix.c
@@ -55,6 +55,13 @@
 #include "qemu/error-report.h"
 #endif
 
+#define PAGE_TOUCH_THREAD_COUNT 8
+typedef struct {
+    char *addr;
+    uint64_t numpages;
+    uint64_t hpagesize;
+} PageRange;
+
 int qemu_get_thread_id(void)
 {
 #if defined(__linux__)
@@ -323,6 +330,52 @@ static void sigbus_handler(int signal)
     siglongjmp(sigjump, 1);
 }
 
+static void *do_touch_pages(void *arg)
+{
+    PageRange *range = (PageRange *)arg;
+    char *start_addr = range->addr;
+    uint64_t numpages = range->numpages;
+    uint64_t hpagesize = range->hpagesize;
+    uint64_t i = 0;
+
+    for (i = 0; i < numpages; i++) {
+        memset(start_addr + (hpagesize * i), 0, 1);
+    }
+    qemu_thread_exit(NULL);
+
+    return NULL;
+}
+
+static int touch_all_pages(char *area, size_t hpagesize, size_t numpages)
+{
+    QemuThread page_threads[PAGE_TOUCH_THREAD_COUNT];
+    PageRange page_range[PAGE_TOUCH_THREAD_COUNT];
+    uint64_t    numpage_per_thread, size_per_thread;
+    int         i = 0, tcount = 0;
+
+    numpage_per_thread = (numpages / PAGE_TOUCH_THREAD_COUNT);
+    size_per_thread = (hpagesize * numpage_per_thread);
+    for (i = 0; i < (PAGE_TOUCH_THREAD_COUNT - 1); i++) {
+        page_range[i].addr = area;
+        page_range[i].numpages = numpage_per_thread;
+        page_range[i].hpagesize = hpagesize;
+
+        qemu_thread_create(page_threads + i, "touch_pages",
+                           do_touch_pages, (page_range + i),
+                           QEMU_THREAD_JOINABLE);
+        tcount++;
+        area += size_per_thread;
+        numpages -= numpage_per_thread;
+    }
+    for (i = 0; i < numpages; i++) {
+        memset(area + (hpagesize * i), 0, 1);
+    }
+    for (i = 0; i < tcount; i++) {
+        qemu_thread_join(page_threads + i);
+    }
+    return 0;
+}
+
 void os_mem_prealloc(int fd, char *area, size_t memory, Error **errp)
 {
     int ret;
@@ -353,9 +406,14 @@ void os_mem_prealloc(int fd, char *area, size_t memory, 
Error **errp)
         size_t hpagesize = qemu_fd_getpagesize(fd);
         size_t numpages = DIV_ROUND_UP(memory, hpagesize);
 
-        /* MAP_POPULATE silently ignores failures */
-        for (i = 0; i < numpages; i++) {
-            memset(area + (hpagesize * i), 0, 1);
+        /* touch pages simultaneously for memory >= 64G */
+        if (memory < (1ULL << 36)) {
+            /* MAP_POPULATE silently ignores failures */
+            for (i = 0; i < numpages; i++) {
+                memset(area + (hpagesize * i), 0, 1);
+            }
+        } else {
+            touch_all_pages(area, hpagesize, numpages);
         }
     }
 
-- 
1.8.3.1