Re: [PATCH] nfs-iostat.py: Print Data Cache Statistics

[Kevin Constantine <Kevin.Constantine-FfNkGbSheRGpB8w63BLUukEOCMrvLtNR@public.gmane.org>]

Chuck Lever wrote:
> Hi Kevin-
> 
> On Apr 21, 2009, at 9:42 PM, Kevin Constantine wrote:
>> I rolled the read cache, and write patches into a single patch below.
>>
>> add --data flag to print data cache statistics
>> print read cache stats from __print_data_cache_stats
>> print stats about bytes written by NFS
>>
>> Signed-off-by: Kevin Constantine <kevin.constantine-FfNkGbSheRGpB8w63BLUukEOCMrvLtNR@public.gmane.org>
>> ---
>> tools/nfs-iostat/nfs-iostat.py |   28 ++++++++++++++++++++++------
>> 1 files changed, 22 insertions(+), 6 deletions(-)
>>
>> diff --git a/tools/nfs-iostat/nfs-iostat.py 
>> b/tools/nfs-iostat/nfs-iostat.py
>> index 9626d42..cb3da59 100644
>> --- a/tools/nfs-iostat/nfs-iostat.py
>> +++ b/tools/nfs-iostat/nfs-iostat.py
>> @@ -220,14 +220,22 @@ class DeviceData:
>>         """Print the data cache hit rate
>>         """
>>         nfs_stats = self.__nfs_data
>> -        app_bytes_read = float(nfs_stats['normalreadbytes'])
>> +        app_bytes_read = float(nfs_stats['normalreadbytes'] + 
>> nfs_stats['directreadbytes'])
> 
> Again, I object to this particular change.  The reason for the current 
> computation is to retain precision and mitigate the chance of an 
> overflow.  Reversing the sense of this computation (by introducing the 
> addition above) increases the probability of overflow here.  Remember 
> that over time, these numbers can grow exceptionally large.
> 
> If you think you need this change, you do need to provide some 
> justification for the reorganization in the patch description.  Why is 
> the current code inadequate?
> 

I've changed my opinion on how the cache hit ratio should be computed and now agree with you.  I was thinking that direct reads should negatively 
impact the ratio, but that doesn't make a whole lot of sense.

>>         if app_bytes_read != 0:
>> -            client_bytes_read = float(nfs_stats['serverreadbytes'] - 
>> nfs_stats['directreadbytes'])
>> -            ratio = ((app_bytes_read - client_bytes_read) * 100) / 
>> app_bytes_read
>> -
>> +            cached_read_bytes = float(app_bytes_read - 
>> float(nfs_stats['serverreadbytes']))
>> +            ratio = (cached_read_bytes * 100) / app_bytes_read
>>             print
>> -            print 'app bytes: %f  client bytes %f' % (app_bytes_read, 
>> client_bytes_read)
>> -            print 'Data cache hit ratio: %4.2f%%' % ratio
> 
> In my opinion, displaying O_DIRECT reads right next to the cache hit 
> ratio is deceptive.  This is why I did not include the O_DIRECT counts 
> here.  They have no impact on the page cache hit ratio, as O_DIRECT data 
> is never cached in the kernel.
> 

I've changed the output considerably this time around.  I think it makes more sense, let me know what you think.

nani:/vol/pamtest mounted on /data/pamtest:

    op/s         rpc bklog
3452.00           13.16
read:             ops/s            kB/s           kB/op         retrans         avg RTT (ms)    avg exe (ms)
                 3452.000        111407.906       32.273        0 (0.0%)           4.633           8.628
write:            ops/s            kB/s           kB/op         retrans         avg RTT (ms)    avg exe (ms)
                   0.000           0.000           0.000        0 (0.0%)           0.000           0.000

            Network                                          Direct I/O             Direct %                  Total
       Read        Write   Cache Reads    Cache Hit %     Read        Write       Read     Write         Read        Write
   107.8750MB     0.0000MB     4.1236MB       3.68%     0.0000MB     0.0000MB     0.00%     0.00%   111.9986MB     0.0000MB


>> +            print '%10s %15s %15s %15s %7s' % ("Data Read:", "From 
>> Server", "From Cache", "Total", "Hit %")
>> +            print '%10s %13.4fMB %13.4fMB %13.4fMB %6.2f%%' % ("", \
>> +                            float(nfs_stats['serverreadbytes']) / 
>> 1024.0 / 1024.0, \
>> +                        cached_read_bytes / 1024.0 / 1024.0, \
>> +                        app_bytes_read / 1024.0 / 1024.0, \
>> +                        ratio)
> 
> If app_bytes_read is zero, we should print a line here that displays 
> zeroes, as is done in other areas of this code.  I know that nfsstat 
> quells output lines containing all zeros, but other tools (like iostat) 
> do not.  What ends up happening is that when there is no I/O activity, 
> no lines are produced at all.  That is confusing to users when 
> nfs-iostat is in the "output one line every N seconds" mode.
> 

Agreed.  It now prints out zeroes.  There are instances where data has been
sent through the NFS stack and counted in the serverbytes counter, but
haven't made it through the userspace stack and haven't been counted so we
get a negative cache hit ratio. In these cases, I'm converting the cache
ratio and cache bytes to zero.

>> +        bytes_written_by_nfs = float(nfs_stats['serverwritebytes'])
>> +    if bytes_written_by_nfs != 0:
>> +        print
>> +            print '%13s %12s' % ("Data Written:", "To Server")
>> +        print '%10s %13.4fMB' % ("", bytes_written_by_nfs / 1024.0 / 
>> 1024.0)
> 
> As you are not dividing by bytes_written_by_nfs, there is no particular 
> reason to gate this output.  I don't see the need for the extra variable 
> here either.  You don't have a similar variable in the read statistics, 
> for example.
> 

I was thinking the gate was there to prevent non-zero values from printing.
This has been fixed.

> You included application and direct read bytes in your read statistics, 
> but not here.  Why not?
> 

The new output prints Network reads/writes, direct reads/writes, total
reads/writes, cache reads, cache ratio and direct i/o ratios for reads/writes.

New Patch below:

add --data flag to print data cache statistics
--data flag prints Network reads/writes, cache reads, cache hit%, Direct I/O reads/writes, Direct I/O% and Total reads/Writes

Signed-off-by: Kevin Constantine <kevin.constantine-FfNkGbSheRGpB8w63BLUukEOCMrvLtNR@public.gmane.org>
---
  tools/nfs-iostat/nfs-iostat.py |   53 +++++++++++++++++++++++++++++++++++++---
  1 files changed, 49 insertions(+), 4 deletions(-)

diff --git a/tools/nfs-iostat/nfs-iostat.py b/tools/nfs-iostat/nfs-iostat.py
index 9626d42..b9f3a96 100644
--- a/tools/nfs-iostat/nfs-iostat.py
+++ b/tools/nfs-iostat/nfs-iostat.py
@@ -220,14 +220,51 @@ class DeviceData:
          """Print the data cache hit rate
          """
          nfs_stats = self.__nfs_data
+	client_bytes_read = 0
+	ratio = 0
+	direct_read_ratio = 0
+	direct_write_ratio = 0
+
          app_bytes_read = float(nfs_stats['normalreadbytes'])
+        client_bytes_read = float(nfs_stats['serverreadbytes'] - nfs_stats['directreadbytes'])
+        direct_read_bytes = float(nfs_stats['directreadbytes'])
+	direct_write_bytes = float(nfs_stats['directwritebytes'])
          if app_bytes_read != 0:
-            client_bytes_read = float(nfs_stats['serverreadbytes'] - nfs_stats['directreadbytes'])
              ratio = ((app_bytes_read - client_bytes_read) * 100) / app_bytes_read
+        if direct_read_bytes != 0:
+            direct_read_ratio = float(nfs_stats['directreadbytes']) * 100 / \
+				(app_bytes_read + direct_read_bytes)
+        if direct_write_bytes != 0:
+            direct_write_ratio = float(nfs_stats['directwritebytes']) * 100 / \
+				(direct_write_bytes + float(nfs_stats['normalwritebytes']))
+
+        network_reads_MB = float(nfs_stats['serverreadbytes']) / 1024.0 / 1024.0
+        network_writes_MB = float(nfs_stats['serverwritebytes']) / 1024.0 / 1024.0
+        cached_reads_MB = float(app_bytes_read - client_bytes_read) / 1024.0 / 1024.0
+        direct_reads_MB = float(nfs_stats['directreadbytes']) / 1024.0 / 1024.0
+        direct_writes_MB = float(nfs_stats['directwritebytes']) / 1024.0 / 1024.0
+        total_reads_MB = float(app_bytes_read + nfs_stats['directreadbytes']) / 1024.0 / 1024.0
+	total_writes_MB = float(nfs_stats['normalwritebytes'] + nfs_stats['directwritebytes']) / \
+			1024.0 / 1024.0
+
+        """ Bytes may have been counted by the NFS stack but haven't been counted by the
+            userspace stack.  When this happens, we see negative cache values.  Set these
+            to zero to avoid confusion.
+        """
+        if cached_reads_MB < 0:
+            cached_reads_MB = 0;
+        if ratio < 0:
+            ratio = 0;

-            print
-            print 'app bytes: %f  client bytes %f' % (app_bytes_read, client_bytes_read)
-            print 'Data cache hit ratio: %4.2f%%' % ratio
+        print
+        print '%18s %51s %20s %22s' % ("Network", "Direct I/O", "Direct %", "Total")
+        print '%10s %12s %13s %14s %8s %12s %10s %9s %12s %12s' % \
+		("Read", "Write", "Cache Reads", "Cache Hit %", "Read", "Write", \
+		"Read", "Write",  "Read", "Write")
+        print '%10.4fMB %10.4fMB %10.4fMB %10.2f%% %10.4fMB %10.4fMB %8.2f%% %8.2f%% %10.4fMB %10.4fMB' % ( \
+		network_reads_MB, network_writes_MB, cached_reads_MB, ratio, direct_reads_MB, \
+	 	direct_writes_MB, direct_read_ratio, direct_write_ratio, total_reads_MB, \
+		total_writes_MB)

      def __print_attr_cache_stats(self, sample_time):
          """Print attribute cache efficiency stats
@@ -390,6 +427,10 @@ class DeviceData:
              self.__print_rpc_op_stats('READ', sample_time)
              self.__print_rpc_op_stats('WRITE', sample_time)
              self.__print_page_stats(sample_time)
+        elif which == 4:
+            self.__print_rpc_op_stats('READ', sample_time)
+            self.__print_rpc_op_stats('WRITE', sample_time)
+            self.__print_data_cache_stats()

  #
  # Functions
@@ -487,6 +528,10 @@ def iostat_command(name):
          if arg in ['-p', '--page']:
              which = 3
              continue
+	
+	if arg in ['--data']:
+	    which = 4
+	    continue

          if arg == sys.argv[0]:
              continue
-- 1.6.2.1