On Mon, Apr 4, 2022 at 5:35 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote: > > On Mon, 4 Apr 2022 16:48:11 +0300 > Tzvetomir Stoyanov <tz.stoyanov@xxxxxxxxx> wrote: > > > > > > Hi Joel, > > > > > That cache file is used for constructing the trace meta-data on the > > > > > guest, before sending it to the host. Usually it is compressed, but it > > > > > could be uncompressed in some cases (depending on the configuration) - > > > > > and in that case it can grow up to a few megabytes. Using memfd is ok > > > > > in most cases, but I'm wondering in the worst case - these few > > > > > megabytes could be a problem, especially if the guest runs with a > > > > > minimum amount of memory. > > > > > > > > > > > > > Can you check that file size on your Android setup with that command, > > > > it will force to not use compression on the guest trace file: > > > > trace-cmd <host trace options> -A <guest> <guest trace options> > > > > --file-version 7 --compression none > > > > > > The file grows to 5.3MB with this. Is this really the common case > > > though? If not, I would still prefer memfd tbh. Is that Ok with you? > > This is meta data right? Which means everything in here is in kernel memory > anyway. kallsyms, events, etc. I do not believe that this will be an issue > even uncompressed. > > > > > > > > > There are two cases that could hit this: > > 1. Using a " --compression none" flag on the guest file. We could > > disable that flag and force trace file v7 always to use compression. I > > cannot imagine a use case for uncompressed trace file, maybe only for > > debug purposes ? > > Please no. I do have some machines that do not have zlib installed. I do > not want to make it a requirement to have zlib for this use case. If we do > not have memory, we could fall back to mktemp. Hi Steven, I'm wondering how you run the latest trace-cmd on those machines ? As these libraries are checked at compile time, this is a compile time dependency - so the trace-cmd compiled with zlib support should not be able to run there ? That's why I think " --compression none" could be used only for debugging. We could implement loading compression libraries dynamically to avoid these compile time dependencyies, or add a compile flag to disable compression support ? > > > 2. If, for some reason, there are no supported compression libraries > > on the guest. Trace-cmd supports libz and libzstd, I believe both are > > widely available. Theoretically it could be possible to have some > > minimalistic VM image without any of these libraries. > > Not theoretical ;-) > > > What are the advantages of using memfd instead of temp file, is it > > only for performance ? Usually collecting trace metadata is not > > performance critical, as it is done before starting the trace. > > I think that the best solution is to use memfd in case the guest file > > is compressed and switch back to a temp file in case of a > > not-compressed file. > > Yeah, we could use memfd if compressed and temp if not. Luckily, this isn't > something exposed across the API so we should be able to experiment with > various implementations, and if people complain about one (like Joel did > for Android), we can try something else. > > -- Steve -- Tzvetomir (Ceco) Stoyanov VMware Open Source Technology Center
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