Re: [Xen-devel] freemem-slack and large memory environments

[Mike Latimer <mlatimer@xxxxxxxx>]

On Friday, February 27, 2015 11:29:12 AM Mike Latimer wrote:
> On Friday, February 27, 2015 08:28:49 AM Mike Latimer wrote:
> After adding 2048aeec, dom0's target is lowered by the required amount (e.g.
> 64GB), but as dom0 cannot balloon down fast enough,
> libxl_wait_for_memory_target returns -5, and the domain create fails
    (wrong return code - libxl_wait_for_memory_target actually returns -3)

With libxl_wait_for_memory_target return code corrected (2048aeec), debug 
messages look like this:

Parsing config from sles12pv
 DBG: start freemem loop
 DBG: free_memkb = 541976, need_memkb = 67651584 (rc=0)
 DBG: dom0_curr_target = 2118976472, set_memory_target = -67109608 (rc=1)
 DBG: wait_for_free_memory = 67651584 (rc=-5)
 DBG: wait_for_memory_target (rc=-3)
failed to free memory for the domain

After failing, dom0 continues to balloon down by the requested amount 
(-67109608), so a subsequent startup attempt would work.

My original fix (2563bca1) was intended to continue looping in freem until dom0 
ballooned down the requested amount. However, this really only worked without 
2048aeec, as wait_for_memory_target was always returning 0. After Stefano 
pointed out this problem, commit 2563bca1 can still be useful - but seems less 
important as ballooning down dom0 is where the major delays are seen.

The following messages show what was happening when wait_for_memory_target was 
always returning 0. I've narrowed it down to just the interesting messages:

DBG: free_memkb = 9794852, need_memkb = 67651584 (rc=0)
DBG: dom0_curr_target = 2118976464, set_memory_target = -67109596 (rc=1)
DBG: dom0_curr_target = 2051866868, set_memory_target = -57856732 (rc=1)
DBG: dom0_curr_target = 1994010136, set_memory_target = -50615004 (rc=1)
DBG: dom0_curr_target = 1943395132, set_memory_target = -43965148 (rc=1)
DBG: dom0_curr_target = 1899429984, set_memory_target = -37538524 (rc=1)
DBG: dom0_curr_target = 1861891460, set_memory_target = -31560412 (rc=1)
DBG: dom0_curr_target = 1830331048, set_memory_target = -25309916 (rc=1)
DBG: dom0_curr_target = 1805021132, set_memory_target = -19514076 (rc=1)
DBG: dom0_curr_target = 1785507056, set_memory_target = -13949660 (rc=1)
DBG: dom0_curr_target = 1771557396, set_memory_target = -8057564 (rc=1)
DBG: dom0_curr_target = 1763499832, set_memory_target = -1862364 (rc=1)

The above situation is no longer relevant, but the overall dom0 target problem 
is still an issue. It now seems rather obvious (hopefully) that the 10 second 
delay in wait_for_memory_target is not sufficient. Should that function be 
modified to monitor ongoing progress and continue waiting as long as progress 
is being made?

Sorry for the long discussion to get to this point. :(

-Mike



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