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SubjectRe: [PATCH v2 2/2]: perf record: enable asynchronous trace writing
Hello,

On Mon, Aug 27, 2018 at 12:33:07PM +0300, Alexey Budankov wrote:
> Hi,
>
> On 27.08.2018 11:38, Jiri Olsa wrote:
> > On Thu, Aug 23, 2018 at 07:47:01PM +0300, Alexey Budankov wrote:
> >
> > SNIP
> >
> >> static int record__mmap_read_evlist(struct record *rec, struct perf_evlist *evlist,
> >> bool overwrite)
> >> {
> >> u64 bytes_written = rec->bytes_written;
> >> - int i;
> >> - int rc = 0;
> >> + int i, rc = 0;
> >> struct perf_mmap *maps;
> >> + int trace_fd = rec->session->data->file.fd;
> >> + struct aiocb **mmap_aio = rec->evlist->mmap_aio;
> >> + int mmap_aio_size = 0;
> >> + off_t off;
> >>
> >> if (!evlist)
> >> return 0;
> >> @@ -546,14 +620,17 @@ static int record__mmap_read_evlist(struct record *rec, struct perf_evlist *evli
> >> if (overwrite && evlist->bkw_mmap_state != BKW_MMAP_DATA_PENDING)
> >> return 0;
> >>
> >> + off = lseek(trace_fd, 0, SEEK_CUR);
> >> +
> >
> > with async write, do we need to query/set the offset like this
> > all the time?
>
> It looks like we need it this way. Internally glibc AIO implements writes
> using pwrite64 syscall in our case. The sycall requires offset as a parameter
> and doesn't update file position on the completion.
>
> >
> > could we just keep/update the offset value in the 'struct perf_data_file'
> > and skip both lseek calls?
>
> Don't see how it is possible. offset is different for every enqeued write
> operation and write areas don't intersect for the whole writing loop.
> To know the final file position it is required to iterate thru
> the loop.

But as far as I can see the offset is linearly updated in
perf_mmap__push() and I guess those two lseek() calls will return
a same value as the last updated offset, no?

Thanks,
Namhyung

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