Avi Kivity wrote:
> David S. Ahern wrote:
>> The short answer is that I am still see large system time hiccups in the
>> guests due to kscand in the guest scanning its active lists. I do see
>> better response for a KVM_MAX_PTE_HISTORY of 3 than with 4. (For
>> completeness I also tried a history of 2, but it performed worse than 3
>> which is no surprise given the meaning of it.)
>>
>>
>> I have been able to scratch out a simplistic program that stimulates
>> kscand activity similar to what is going on in my real guest (see
>> attached). The program requests a memory allocation, initializes it (to
>> get it backed) and then in a loop sweeps through the memory in chunks
>> similar to a program using parts of its memory here and there but
>> eventually accessing all of it.
>>
>> Start the RHEL3/CentOS 3 guest with *2GB* of RAM (or more). The key is
>> using a fair amount of highmem. Start a couple of instances of the
>> attached. For example, I've been using these 2:
>>
>>     memuser 768M 120 5 300
>>     memuser 384M 300 10 600
>>
>> Together these instances take up a 1GB of RAM and once initialized
>> consume very little CPU. On kvm they make kscand and kswapd go nuts
>> every 5-15 minutes. For comparison, I do not see the same behavior for
>> an identical setup running on esx 3.5.
>>   
> 
> I haven't been able to reproduce this:
> 
>> [root@localhost root]# ps -elf | grep -E 'memuser|kscand'
>> 1 S root         7     1  1  75   0    -     0 schedu 10:07 ?       
>> 00:00:26 [kscand]
>> 0 S root      1464     1  1  75   0    - 196986 schedu 10:20 pts/0  
>> 00:00:21 ./memuser 768M 120 5 300
>> 0 S root      1465     1  0  75   0    - 98683 schedu 10:20 pts/0   
>> 00:00:10 ./memuser 384M 300 10 600
>> 0 S root      2148  1293  0  75   0    -   922 pipe_w 10:48 pts/0   
>> 00:00:00 grep -E memuser|kscand
> 
> The workload has been running for about half an hour, and kswapd cpu
> usage doesn't seem significant.  This is a 2GB guest running with my
> patch ported to kvm.git HEAD.  Guest is has 2G of memory.
> 

I'm running on the per-page-pte-tracking branch, and I am still seeing it. 

I doubt you want to sit and watch the screen for an hour, so install sysstat if not already, change the sample rate to 1 minute (/etc/cron.d/sysstat), let the server run for a few hours and then run 'sar -u'. You'll see something like this:

10:12:11 AM       LINUX RESTART

10:13:03 AM       CPU     %user     %nice   %system   %iowait     %idle
10:14:01 AM       all      0.08      0.00      2.08      0.35     97.49
10:15:03 AM       all      0.05      0.00      0.79      0.04     99.12
10:15:59 AM       all      0.15      0.00      1.52      0.06     98.27
10:17:01 AM       all      0.04      0.00      0.69      0.04     99.23
10:17:59 AM       all      0.01      0.00      0.39      0.00     99.60
10:18:59 AM       all      0.00      0.00      0.12      0.02     99.87
10:20:02 AM       all      0.18      0.00     14.62      0.09     85.10
10:21:01 AM       all      0.71      0.00     26.35      0.01     72.94
10:22:02 AM       all      0.67      0.00     10.61      0.00     88.72
10:22:59 AM       all      0.14      0.00      1.80      0.00     98.06
10:24:03 AM       all      0.13      0.00      0.50      0.00     99.37
10:24:59 AM       all      0.09      0.00     11.46      0.00     88.45
10:26:03 AM       all      0.16      0.00      0.69      0.03     99.12
10:26:59 AM       all      0.14      0.00     10.01      0.02     89.83
10:28:03 AM       all      0.57      0.00      2.20      0.03     97.20
Average:          all      0.21      0.00      5.55      0.05     94.20


every one of those jumps in %system time directly correlates to kscand activity. Without the memuser programs running the guest %system time is <1%. The point of this silly memuser program is just to use high memory -- let it age, then make it active again, sit idle, repeat. If you run kvm_stat with -l in the host you'll see the jump in pte writes/updates. An intern here added a timestamp to the kvm_stat output for me which helps to directly correlate guest/host data.


I also ran my real guest on the branch. Performance at boot through the first 15 minutes was much better, but I'm still seeing recurring hits every 5 minutes when kscand kicks in. Here's the data from the guest for the first one which happened after 15 minutes of uptime:

active_anon_scan: HighMem, age 11, count[age] 24886 -> 5796, direct 24845, dj 59

active_anon_scan: HighMem, age 7, count[age] 47772 -> 21289, direct 40868, dj 103

active_anon_scan: HighMem, age 3, count[age] 91007 -> 329, direct 45805, dj 1212

The kvm_stat data for this time period is attached due to line lengths.


Also, I forgot to mention this before, but there is a bug in the kscand code in the RHEL3U8 kernel. When it scans the cache list it uses the count from the anonymous list:

            if (need_active_cache_scan(zone)) {
                for (age = MAX_AGE-1; age >= 0; age--)  {
                    scan_active_list(zone, age,
                        &zone->active_cache_list[age],
                        zone->active_anon_count[age]);
                              ^^^^^^^^^^^^^^^^^
                    if (current->need_resched)
                        schedule();
                }
            }

When the anonymous count is higher it is scanning the cache list repeatedly. An example of that was captured here:

active_cache_scan: HighMem, age 7, count[age] 222 -> 179, count anon 111967, direct 626, dj 3

count anon is active_anon_count[age] which at this moment was 111,967. There were only 222 entries in the cache list, but the count value passed to scan_active_list was 111,967. When the cache list has a lot of direct pages, that causes a larger hit on kvm than needed. That said, I have to live with the bug in the guest.

david