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Wed, 31 Jan 2024 13:15:00 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id Jrz+F9RHumXrAwAAD6G6ig (envelope-from ); Wed, 31 Jan 2024 13:15:00 +0000 From: Fabiano Rosas To: Peter Xu Cc: qemu-devel@nongnu.org, Hao Xiang , Yuan Liu , Bryan Zhang , Avihai Horon Subject: Re: [PATCH 1/5] migration/multifd: Separate compression ops from non-compression In-Reply-To: References: <20240126221943.26628-1-farosas@suse.de> <20240126221943.26628-2-farosas@suse.de> <87mssoe2fj.fsf@suse.de> <8734uedff0.fsf@suse.de> Date: Wed, 31 Jan 2024 10:14:58 -0300 Message-ID: <875xz9lk4t.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain Authentication-Results: smtp-out2.suse.de; none X-Spamd-Result: default: False [-4.29 / 50.00]; ARC_NA(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; BAYES_HAM(-3.00)[100.00%]; FROM_HAS_DN(0.00)[]; TO_DN_SOME(0.00)[]; TO_MATCH_ENVRCPT_ALL(0.00)[]; NEURAL_HAM_LONG(-1.00)[-1.000]; MIME_GOOD(-0.10)[text/plain]; RCPT_COUNT_FIVE(0.00)[6]; RCVD_COUNT_THREE(0.00)[3]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; NEURAL_HAM_SHORT(-0.19)[-0.954]; FUZZY_BLOCKED(0.00)[rspamd.com]; FROM_EQ_ENVFROM(0.00)[]; MIME_TRACE(0.00)[0:+]; RCVD_TLS_ALL(0.00)[]; MID_RHS_MATCH_FROM(0.00)[] Received-SPF: pass client-ip=195.135.223.131; envelope-from=farosas@suse.de; helo=smtp-out2.suse.de X-Spam_score_int: -43 X-Spam_score: -4.4 X-Spam_bar: ---- X-Spam_report: (-4.4 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_MED=-2.3, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Peter Xu writes: > On Tue, Jan 30, 2024 at 12:11:47PM -0300, Fabiano Rosas wrote: >> Peter Xu writes: >> >> > On Mon, Jan 29, 2024 at 09:42:24AM -0300, Fabiano Rosas wrote: >> >> Peter Xu writes: >> >> >> >> > On Fri, Jan 26, 2024 at 07:19:39PM -0300, Fabiano Rosas wrote: >> >> >> +static MultiFDMethods multifd_socket_ops = { >> >> >> + .send_setup = multifd_socket_send_setup, >> >> >> + .send_cleanup = multifd_socket_send_cleanup, >> >> >> + .send_prepare = multifd_socket_send_prepare, >> >> > >> >> > Here it's named with "socket", however not all socket-based multifd >> >> > migrations will go into this route, e.g., when zstd compression enabled it >> >> > will not go via this route, even if zstd also uses sockets as transport. >> >> > From that pov, this may be slightly confusing. Maybe it suites more to be >> >> > called "socket_plain" / "socket_no_comp"? >> >> > >> >> > One step back, I had a feeling that the current proposal tried to provide a >> >> > single ->ops to cover a model where we may need more than one layer of >> >> > abstraction. >> >> > >> >> > Since it might be helpful to allow multifd send arbitrary data (e.g. for >> >> > VFIO? Avihai might have an answer there..), I'll try to even consider that >> >> > into the picture. >> >> > >> >> > Let's consider the ultimate goal of multifd, where the simplest model could >> >> > look like this in my mind (I'm only discussing sender side, but it'll be >> >> > similar on recv side): >> >> > >> >> > prepare() send() >> >> > Input ----------------> IOVs ------------> iochannels >> >> > >> >> > [I used prepare/send, but please think them as generic terms, not 100% >> >> > aligned with what we have with existing multifd_ops, or what you proposed >> >> > later] >> >> > >> >> > Here what are sure, IMHO, is: >> >> > >> >> > - We always can have some input data to dump; I didn't use "guest pages" >> >> > just to say we may allow arbitrary data. For any multifd user that >> >> > would like to dump arbitrary data, they can already provide IOVs, so >> >> > here input can be either "MultiFDPages_t" or "IOVs". >> >> >> >> Or anything else, since the client code also has control over send(), >> >> no? So it could give multifd a pointer to some memory and then use >> >> send() to do whatever it wants with it. Multifd is just providing worker >> >> threads and "scheduling". >> > >> > IOVs contain the case of one single buffer, where n_iovs==1. Here I >> > mentioned IOVs explicitly because I want to make it part of the protocol so >> > that the interface might be clearer, on what is not changing, and what can >> > change for a multifd client. >> >> Got it. I agree. >> >> >> >> >> Also note that multifd clients currently _do not_ provide IOVs. They >> >> merely provide data to multifd (p->pages) and then convert that data >> >> into IOVs at prepare(). This is different, because multifd currently >> >> holds that p->pages (and turns that into p->normal), which means the >> >> client code does not need to store the data across iterations (in the >> >> case of RAM which is iterative). >> > >> > They provide? AFAIU that's exactly MultiFDSendParams.iov as of now, while >> > iov_nums is the length. >> >> Before that, the ram code needs to pass in the p->pages->offset array >> first. Then, that gets put into p->normal. Then, that gets put into >> p->iov at prepare(). So it's not a simple "fill p->iov and pass it to >> multifd". >> >> Hmm, could we just replace multifd_send_state->pages with a >> multifd_send_state->iov? I don't really understand why do we need to >> carry that pages->offset around. > > I am thinking the p->normal is mostly redundant.. at least on the sender > side that I just read. Since I'll be preparing a new spin of the multifd > cleanup series I posted, maybe I can append one more to try dropping > p->normal[] completely. Just for reference, you don't have to use it, but I have this patch: https://gitlab.com/farosas/qemu/-/commit/4316e145ae7e7bf378ef7fde64c2b02260362847 >> >> >> >> >> > >> >> > - We may always want to have IOVs to represent the buffers at some point, >> >> > no matter what the input it >> >> > >> >> > - We always flush the IOVs to iochannels; basically I want to say we can >> >> > always assume the last layer is connecting to QIOChannel APIs, while I >> >> > don't think there's outliers here so far, even if the send() may differ. >> >> > >> >> > Then _maybe_ it's clearer that we can have two layers of OPs? >> >> > >> >> > - prepare(): it tells how the "input" will be converted into a scatter >> >> > gatter list of buffers. All compression methods fall into this afaiu. >> >> > This has _nothing_ to do on how the buffers will be sent. For >> >> > arbitrary-typed input, this can already be a no-op since the IOVs >> >> > provided can already be passed over to send(). >> >> > >> >> > - send(): how to dump the IOVs to the iochannels. AFAIU this is motly >> >> > only useful for fixed-ram migrations. >> >> > >> >> > Would this be clearer, rather than keep using a single multifd_ops? >> >> >> >> Sorry, I don't see how what you describe is any different than what we >> >> have. And I don't see how any of this would mean more than one >> >> multifd_ops. We already have multifd_ops->prepare() and >> >> multifd_ops->send(). What am I missing? >> > >> > I meant instead of having a single MultiFDMethods, we can have >> > MultifdPrepareOps and MultifdSendOps separately. >> > >> > Now with single MultiFDMethods, it must always provide e.g. both prepare() >> > and send() in one set of OPs for one use case. What I wanted to say is >> > maybe it is cleaner we split it into two OPs, then all the socket-based >> > scenarios can already stick with the same send() method, even though they >> > can prepare() differently. >> >> Hmm, so zlib/zstd implement all ops except for the send one. And >> socket_plain and file implement all prepare hooks plus the send. So we'd >> have sort of a data handling layer and a transport layer. I'll see how >> it looks. > > Yeah something like that if you agree; I'd think socket_plain can also use > the same socket send() with all the rest? Again, I don't see its specialty > except the zero copy possibility, while the latter should be able to be > covered by proper setup of p->write_flags. > I see. Makes sense. >> >> > >> > IOW, for this base patchset to pave way for compression accelerators, IIUC >> > we don't need a send() yet so far? Should they still work pretty well with >> > qio_channel_writev_full_all() with proper touchups on p->write_flags just >> > for zero copy purposes? >> >> Yes. The point here is to just give everyone a heads-up so we avoid >> changing the code in incompatible ways. >> >> > >> > I'll have a read again to your previous multifd-packet-cleanups branch I >> > guess. but this series definitely doesn't apply there already. >> >> multifd-packet-cleanups attempts to replace MultiFDPages_t with a >> generic data structure. That's a separate issue. >>