[Qemu-devel] Re: [PATCH 09/10] Exit loop if we have been there too long

[Juan Quintela]

Avi Kivity <address@hidden> wrote:
> On 12/01/2010 03:52 AM, Juan Quintela wrote:
>> >   - 512GB guest is really the target?
>>
>> no, problems exist with smaller amounts of RAM.  with 16GB guest it is
>> trivial to get 1s stalls, 64GB guest, 3-4s, with more memory, migration
>> is flaky to say the less.
>>
>> >   - how much cpu time can we use for these things?
>>
>> the problem here is that we are forced to walk the bitmap too many
>> times, we want to do it less times.
>
> How much time is spent walking bitmaps?  Are you sure this is the problem?

see my 10/10 patch, that one makes ram_save_live() to move from 80% of the
time in the profile to the second one with ~5% or so.

The important bit is:

 static uint64_t ram_save_remaining(void)
 {
-    RAMBlock *block;
-    uint64_t count = 0;
-
-    QLIST_FOREACH(block, &ram_list.blocks, next) {
-        ram_addr_t addr;
-        for (addr = block->offset; addr < block->offset + block->length;
-             addr += TARGET_PAGE_SIZE) {
-            if (cpu_physical_memory_get_dirty(addr, MIGRATION_DIRTY_FLAG)) {
-                count++;
-            }
-        }
-    }
-
-    return count;
+    return ram_list.dirty_pages;
 }


We dont' need to walk all bitmap to see how much memory is remaining.

syncing the bitmap is also expensive, but just now we have other issues
that hide it.  That is the reason that I didn't started trynig to get a
better interface with kvm, 1st I will try to remove the bigger
bottlenecks.

>> >   - how many dirty pages do we have to care?
>>
>> default values and assuming 1Gigabit ethernet for ourselves ~9.5MB of
>> dirty pages to have only 30ms of downtime.
>
> 1Gb/s * 30ms = 100 MB/s * 30 ms = 3 MB.

I will learn to make math with time and bytes at some point O:-)

Later, Juan.