linux-kernel - Re: [PATCH v5 07/10] perf record: implement -z,--compression

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Date:   Thu, 7 Mar 2019 11:26:16 +0300
From:   Alexey Budankov <alexey.budankov@...ux.intel.com>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 07/10] perf record: implement -z,--compression_level=n
 option and compression


On 05.03.2019 15:26, Jiri Olsa wrote:
> On Fri, Mar 01, 2019 at 06:58:32PM +0300, Alexey Budankov wrote:
> 
> SNIP
> 
>> +static size_t record__process_comp_header(void *record, size_t increment)
>> +{
>> +	struct compressed_event *event = record;
>> +	size_t size = sizeof(struct compressed_event);
>> +
>> +	if (increment) {
>> +		event->header.size += increment;
>> +		return increment;
>> +	} else {
>> +		event->header.type = PERF_RECORD_COMPRESSED;
>> +		event->header.size = size;
>> +		return size;
>> +	}
> 
> so zstd_compress_stream_to_records calls this in the loop:
> 
>           while (input.pos < input.size) {
> 	  	...
>                 size = process_header(record, 0);
> 		...
>                 size = process_header(record, size);
> 	}
> 
> the header.size will get overwritten with every new iteration, no?

Every new record is headed by PERF_RECORD_COMPRESSED header an updated with 
resulted compressed frame size after compression. 

process_header(, 0) puts header, then 
compression puts compressed frame, then 
process_header(, N) updates header.size value (+=).

~Alexey

> 
> jirka
>