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XROOTD-L  March 2020

XROOTD-L March 2020

Subject:

Re: Meaning of memory cacheing log stats

From:

Andrew Hanushevsky <[log in to unmask]>

Reply-To:

Support use of xrootd by HEP experiments <[log in to unmask]>

Date:

Wed, 4 Mar 2020 13:48:56 -0800

Content-Type:

TEXT/PLAIN

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Parts/Attachments

TEXT/PLAIN (103 lines)

Hi Thomas,

I don't have a real answer for that. Obviously, some data had to be read 
into the cache. I seems to be the way "read into cache" is calculate and 
is sensitive to the actual read size. So, I guess you could classify it as 
a bug. The other statistics seem OK.

Andy

On Wed, 4 Mar 2020, Thomas Byrne - UKRI STFC wrote:

> Hi,
>
> Thanks for this Andy, it's very helpful.
>
> I'm confused why 'Read' is *always* zero on our cache log stats, I would have expected it to be a multiple of the pagesize. Have I got the wrong end of the stick here?
>
> Cheers
> Tom
>
>> -----Original Message-----
>> From: [log in to unmask] <[log in to unmask]> On
>> Behalf Of Andrew Hanushevsky
>> Sent: 02 March 2020 23:59
>> To: Patargias, George (STFC,RAL,SC) <[log in to unmask]>
>> Cc: [log in to unmask]
>> Subject: Re: Meaning of memory cacheing log stats
>>
>> Hi George,
>>
>> So, here it is:
>>
>>      0 Read;   Total number of bytes read into the cache
>> 38581 Get;    Number of bytes delivered from the cache
>>      0 Pass;   Number of bytes read but not cached
>>      0 Write;  Total number of bytes written from the cache
>>      0 Put;    Number of bytes updated in the cache
>>     20 Hits;   Number of times wanted data was in the cache
>>      3 Miss;   Number of times wanted data was *not* in the cache
>>      0 pead;   Bytes read via preread (not included in BytesRead)
>>      0 HitsPR; Number of pages of   wanted data was just preread
>>      0 MissPR  Number of pages of unwanted data was just preread
>>
>> So, it would seem the cache is being effective as number of hits is far greater
>> than the number of misses.
>>
>> Andy
>>
>>
>> On Mon, 2 Mar 2020, George Patargias - UKRI STFC wrote:
>>
>>> Hello,
>>>
>>>
>>> We have turned on the printing of mem cache statistics using the
>>> "logstasts" flag of the pss.cache
>>>
>>> directive as shown below
>>>
>>>
>>> pss.cache logstats max2cache 33554432 pagesize 33554432 size
>>> 17179869184
>>>
>>>
>>> Can you please explain briefly what is the meaning of the numbers
>> appearing in the corresponding lines of the xrootd log? The following is a
>> common example we see on our xrootd server but it is not clear whether the
>> cache is being effective in this case (we suspect it isn't).
>>>
>>>
>>> Cache: Stats: 0 Read; 38581 Get; 0 Pass; 0 Write; 0 Put; 20 Hits; 3
>> Miss; 0 pead; 0 HitsPR; 0 MissPR; Path
>>>
>>>
>>> Thanks,
>>>
>>>
>>> George
>>>
>>>
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