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From: Steven Rostedt <rostedt@goodmis.org>
To: Tzvetomir Stoyanov <tz.stoyanov@gmail.com>
Cc: Linux Trace Devel <linux-trace-devel@vger.kernel.org>
Subject: Re: [PATCH v6 17/45] trace-cmd library: Add support for zlib compression library
Date: Tue, 22 Jun 2021 09:31:13 -0400	[thread overview]
Message-ID: <20210622093113.0e5c0384@gandalf.local.home> (raw)
In-Reply-To: <CAPpZLN5oB30ZUKkbeFuLhPuigMPzi4AT7P7mPTYd1AuHXXVuhw@mail.gmail.com>

On Tue, 22 Jun 2021 13:29:36 +0300
Tzvetomir Stoyanov <tz.stoyanov@gmail.com> wrote:

> On Tue, Jun 22, 2021 at 4:26 AM Steven Rostedt <rostedt@goodmis.org> wrote:
> >
> > On Mon, 14 Jun 2021 10:50:01 +0300
> > "Tzvetomir Stoyanov (VMware)" <tz.stoyanov@gmail.com> wrote:
> >  
> > > If libz is available, use that library to provide trace file compression
> > > support. The library is detected runtime.  
> >
> > Why have the library detected at runtime?
> >
> > If it is detected, we can then have the library flags include -lz.
> >
> > Why use dlopen to load zlib? And not just include it?
> >
> > This seems rather fragile to try to get right.  
> 
> The idea of this design is not to bring additional mandatory
> dependencies. I do not know if libz is available by default, but even
> if we assume it is - each additional compression library that is added
> will be a mandatory dependency. The compression is not a mandatory
> functionality, trace-cmd can work without it. Why do you think it is
> fragile, I think dlopen() uses the same core linker logic to find and
> load the library. The only difference I see is that using "-lz" leads
> to a mandatory dependency, trace-cmd will not run without it.
> 

The dependency is determined at compile time, just like we do for the
python libraries, and dwarf, and anything else that might add a new
feature.

Let me explain a scenario of why I called it fragile. Let's say that
trace-cmd has no package dependency on zlib, but the system has some other
package that does have a dependency. Thus the package manager pulls in zlib
to satisfy this other package.

The user does a recording, and trace-cmd detects zlib, and compresses the
data.

Later, the user decides they do not need this other package and uninstalls
it. The package manager sees there's nothing that depends on zlib anymore,
and uninstalls zlib as well.

Then the user goes to read their trace.dat file, and suddenly trace-cmd
can't read it!

THAT is what I call fragile. And if something like that ever happened to
me, I would stop using whatever did that to me.

Manually pulling in system dynamic libraries with dlopen is something I
never heard of. The way to do this is make it a build time dependency. If
zlib exists, then define HAVE_ZLIB and allow compressions and everything
else. If it does not, then we don't support compression. Simple as that.

Making it a runtime dependency has a lot of issues, *especially* since one
run of trace-cmd (the reporting) depends on a previous run of trace-cmd
(the recording), and if the environment changes between the two, the user
will rightfully say WTF!

-- Steve

  reply	other threads:[~2021-06-22 13:31 UTC|newest]

Thread overview: 64+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-14  7:49 [PATCH v6 00/45] Add trace file compression Tzvetomir Stoyanov (VMware)
2021-06-14  7:49 ` [PATCH v6 01/45] trace-cmd library: Remove unused private APIs for creating trace files Tzvetomir Stoyanov (VMware)
2021-06-14  7:49 ` [PATCH v6 02/45] trace-cmd library: Remove unused API tracecmd_update_option Tzvetomir Stoyanov (VMware)
2021-06-14  7:49 ` [PATCH v6 03/45] trace-cmd: Check if file version is supported Tzvetomir Stoyanov (VMware)
2021-06-21 22:27   ` Steven Rostedt
2021-06-21 22:36     ` Steven Rostedt
2021-06-14  7:49 ` [PATCH v6 04/45] trace-cmd library: Add new API to get file version of input handler Tzvetomir Stoyanov (VMware)
2021-06-14  7:49 ` [PATCH v6 05/45] trace-cmd library: Select the file version when writing trace file Tzvetomir Stoyanov (VMware)
2021-06-14  7:49 ` [PATCH v6 06/45] trace-cmd: Add APIs for library initialization and free Tzvetomir Stoyanov (VMware)
2021-06-14  7:49 ` [PATCH v6 07/45] trace-cmd library: Add support for compression algorithms Tzvetomir Stoyanov (VMware)
2021-06-14  7:49 ` [PATCH v6 08/45] trace-cmd list: Show supported " Tzvetomir Stoyanov (VMware)
2021-06-14  7:49 ` [PATCH v6 09/45] trace-cmd library: Bump the trace file version to 7 Tzvetomir Stoyanov (VMware)
2021-06-14  7:49 ` [PATCH v6 10/45] trace-cmd library: Compress part of the trace file Tzvetomir Stoyanov (VMware)
2021-06-14  7:49 ` [PATCH v6 11/45] trace-cmd library: Read compressed " Tzvetomir Stoyanov (VMware)
2021-06-14  7:49 ` [PATCH v6 12/45] trace-cmd library: Add new API to get compression of input handler Tzvetomir Stoyanov (VMware)
2021-06-14  7:49 ` [PATCH v6 13/45] trace-cmd library: Inherit compression algorithm from input file Tzvetomir Stoyanov (VMware)
2021-06-14  7:49 ` [PATCH v6 14/45] trace-cmd library: Extend the create file APIs to support different compression Tzvetomir Stoyanov (VMware)
2021-06-14  7:49 ` [PATCH v6 15/45] trace-cmd record: Add new parameter --compression Tzvetomir Stoyanov (VMware)
2021-06-14  7:50 ` [PATCH v6 16/45] trace-cmd dump: Add support for trace files version 7 Tzvetomir Stoyanov (VMware)
2021-06-14  7:50 ` [PATCH v6 17/45] trace-cmd library: Add support for zlib compression library Tzvetomir Stoyanov (VMware)
2021-06-22  1:26   ` Steven Rostedt
2021-06-22 10:29     ` Tzvetomir Stoyanov
2021-06-22 13:31       ` Steven Rostedt [this message]
2021-06-14  7:50 ` [PATCH v6 18/45] trace-cmd library: Hide the logic for updating buffer offset Tzvetomir Stoyanov (VMware)
2021-06-22  2:10   ` Steven Rostedt
2021-06-14  7:50 ` [PATCH v6 19/45] trace-cmd: Move buffers description outside of options Tzvetomir Stoyanov (VMware)
2021-06-21 23:07   ` Steven Rostedt
2021-06-14  7:50 ` [PATCH v6 20/45] trace-cmd library: Track the offset in the option section in the trace file Tzvetomir Stoyanov (VMware)
2021-06-14  7:50 ` [PATCH v6 21/45] trace-cmd library: Add compression of the option section of " Tzvetomir Stoyanov (VMware)
2021-06-21 23:10   ` Steven Rostedt
2021-06-22 10:43     ` Tzvetomir Stoyanov
2021-06-14  7:50 ` [PATCH v6 22/45] trace-cmd library: Refactor the logic for writing trace data in the file Tzvetomir Stoyanov (VMware)
2021-06-21 23:12   ` Steven Rostedt
2021-06-14  7:50 ` [PATCH v6 23/45] trace-cmd library: Add APIs for read and write compressed data in chunks Tzvetomir Stoyanov (VMware)
2021-06-14  7:50 ` [PATCH v6 24/45] trace-cmd: Compress trace data Tzvetomir Stoyanov (VMware)
2021-06-14  7:50 ` [PATCH v6 25/45] trace-cmd: Read compressed " Tzvetomir Stoyanov (VMware)
2021-06-21 23:23   ` Steven Rostedt
2021-06-22 10:50     ` Tzvetomir Stoyanov
2021-06-22 13:51       ` Steven Rostedt
2021-06-14  7:50 ` [PATCH v6 26/45] trace-cmd library: Compress latency " Tzvetomir Stoyanov (VMware)
2021-06-14  7:50 ` [PATCH v6 27/45] trace-cmd: Read compressed " Tzvetomir Stoyanov (VMware)
2021-06-14  7:50 ` [PATCH v6 28/45] trace-cmd library: Reuse within the library the function that checks file state Tzvetomir Stoyanov (VMware)
2021-06-14  7:50 ` [PATCH v6 29/45] trace-cmd library: Make tracecmd_copy_headers() to work with output handler Tzvetomir Stoyanov (VMware)
2021-06-14  7:50 ` [PATCH v6 30/45] trace-cmd: Do not use trace file compression with streams Tzvetomir Stoyanov (VMware)
2021-06-14  7:50 ` [PATCH v6 31/45] trace-cmd library: Add new API to get file version of output handler Tzvetomir Stoyanov (VMware)
2021-06-14  7:50 ` [PATCH v6 32/45] trace-cmd: Add file state parameter to tracecmd_copy Tzvetomir Stoyanov (VMware)
2021-06-14  7:50 ` [PATCH v6 33/45] trace-cmd: Copy CPU count in tracecmd_copy Tzvetomir Stoyanov (VMware)
2021-06-21 23:27   ` Steven Rostedt
2021-06-14  7:50 ` [PATCH v6 34/45] trace-cmd: Copy buffers description " Tzvetomir Stoyanov (VMware)
2021-06-14  7:50 ` [PATCH v6 35/45] trace-cmd: Copy options " Tzvetomir Stoyanov (VMware)
2021-06-14  7:50 ` [PATCH v6 36/45] trace-cmd library: Refactor the logic for writing CPU trace data Tzvetomir Stoyanov (VMware)
2021-06-14  7:50 ` [PATCH v6 37/45] trace-cmd library: Refactor the logic for writing CPU instance " Tzvetomir Stoyanov (VMware)
2021-06-14  7:50 ` [PATCH v6 38/45] trace-cmd: Copy trace data in tracecmd_copy Tzvetomir Stoyanov (VMware)
2021-06-14  7:50 ` [PATCH v6 39/45] trace-cmd: Add compression parameter to tracecmd_copy Tzvetomir Stoyanov (VMware)
2021-06-14  7:50 ` [PATCH v6 40/45] trace-cmd: Add new command "trace-cmd convert" Tzvetomir Stoyanov (VMware)
2021-06-14  7:50 ` [PATCH v6 41/45] trace-cmd record: Update man page Tzvetomir Stoyanov (VMware)
2021-06-14  7:50 ` [PATCH v6 42/45] trace-cmd: Add convert " Tzvetomir Stoyanov (VMware)
2021-06-14  7:50 ` [PATCH v6 43/45] trace-cmd: Update bash completion Tzvetomir Stoyanov (VMware)
2021-06-14  7:50 ` [PATCH v6 44/45] trace-cmd list: Update the man page Tzvetomir Stoyanov (VMware)
2021-06-14  7:50 ` [PATCH v6 45/45] trace-cmd: Update trace.dat " Tzvetomir Stoyanov (VMware)
2021-06-22  0:37   ` Steven Rostedt
2021-06-22 11:05     ` Tzvetomir Stoyanov
2021-06-22 13:58       ` Steven Rostedt
2021-06-22  2:22 ` [PATCH v6 00/45] Add trace file compression Steven Rostedt

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