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Wed, 27 Aug 2025 10:23:05 -0700 (PDT) Message-ID: <3cf8eef1-04e6-44b9-ad41-144de9cbf1d7@citrix.com> Date: Wed, 27 Aug 2025 18:23:03 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] Optimise restore memory allocation To: Frediano Ziglio , xen-devel@lists.xenproject.org Cc: Anthony PERARD , Juergen Gross , =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= , Oleksii Kurochko References: <20250827123309.39699-1-frediano.ziglio@cloud.com> Content-Language: en-GB From: Andrew Cooper Autocrypt: addr=andrew.cooper3@citrix.com; keydata= xsFNBFLhNn8BEADVhE+Hb8i0GV6mihnnr/uiQQdPF8kUoFzCOPXkf7jQ5sLYeJa0cQi6Penp VtiFYznTairnVsN5J+ujSTIb+OlMSJUWV4opS7WVNnxHbFTPYZVQ3erv7NKc2iVizCRZ2Kxn srM1oPXWRic8BIAdYOKOloF2300SL/bIpeD+x7h3w9B/qez7nOin5NzkxgFoaUeIal12pXSR Q354FKFoy6Vh96gc4VRqte3jw8mPuJQpfws+Pb+swvSf/i1q1+1I4jsRQQh2m6OTADHIqg2E ofTYAEh7R5HfPx0EXoEDMdRjOeKn8+vvkAwhviWXTHlG3R1QkbE5M/oywnZ83udJmi+lxjJ5 YhQ5IzomvJ16H0Bq+TLyVLO/VRksp1VR9HxCzItLNCS8PdpYYz5TC204ViycobYU65WMpzWe LFAGn8jSS25XIpqv0Y9k87dLbctKKA14Ifw2kq5OIVu2FuX+3i446JOa2vpCI9GcjCzi3oHV e00bzYiHMIl0FICrNJU0Kjho8pdo0m2uxkn6SYEpogAy9pnatUlO+erL4LqFUO7GXSdBRbw5 gNt25XTLdSFuZtMxkY3tq8MFss5QnjhehCVPEpE6y9ZjI4XB8ad1G4oBHVGK5LMsvg22PfMJ ISWFSHoF/B5+lHkCKWkFxZ0gZn33ju5n6/FOdEx4B8cMJt+cWwARAQABzSlBbmRyZXcgQ29v cGVyIDxhbmRyZXcuY29vcGVyM0BjaXRyaXguY29tPsLBegQTAQgAJAIbAwULCQgHAwUVCgkI CwUWAgMBAAIeAQIXgAUCWKD95wIZAQAKCRBlw/kGpdefoHbdD/9AIoR3k6fKl+RFiFpyAhvO 59ttDFI7nIAnlYngev2XUR3acFElJATHSDO0ju+hqWqAb8kVijXLops0gOfqt3VPZq9cuHlh IMDquatGLzAadfFx2eQYIYT+FYuMoPZy/aTUazmJIDVxP7L383grjIkn+7tAv+qeDfE+txL4 SAm1UHNvmdfgL2/lcmL3xRh7sub3nJilM93RWX1Pe5LBSDXO45uzCGEdst6uSlzYR/MEr+5Z JQQ32JV64zwvf/aKaagSQSQMYNX9JFgfZ3TKWC1KJQbX5ssoX/5hNLqxMcZV3TN7kU8I3kjK mPec9+1nECOjjJSO/h4P0sBZyIUGfguwzhEeGf4sMCuSEM4xjCnwiBwftR17sr0spYcOpqET ZGcAmyYcNjy6CYadNCnfR40vhhWuCfNCBzWnUW0lFoo12wb0YnzoOLjvfD6OL3JjIUJNOmJy RCsJ5IA/Iz33RhSVRmROu+TztwuThClw63g7+hoyewv7BemKyuU6FTVhjjW+XUWmS/FzknSi dAG+insr0746cTPpSkGl3KAXeWDGJzve7/SBBfyznWCMGaf8E2P1oOdIZRxHgWj0zNr1+ooF /PzgLPiCI4OMUttTlEKChgbUTQ+5o0P080JojqfXwbPAyumbaYcQNiH1/xYbJdOFSiBv9rpt TQTBLzDKXok86M7BTQRS4TZ/ARAAkgqudHsp+hd82UVkvgnlqZjzz2vyrYfz7bkPtXaGb9H4 Rfo7mQsEQavEBdWWjbga6eMnDqtu+FC+qeTGYebToxEyp2lKDSoAsvt8w82tIlP/EbmRbDVn 7bhjBlfRcFjVYw8uVDPptT0TV47vpoCVkTwcyb6OltJrvg/QzV9f07DJswuda1JH3/qvYu0p vjPnYvCq4NsqY2XSdAJ02HrdYPFtNyPEntu1n1KK+gJrstjtw7KsZ4ygXYrsm/oCBiVW/OgU g/XIlGErkrxe4vQvJyVwg6YH653YTX5hLLUEL1NS4TCo47RP+wi6y+TnuAL36UtK/uFyEuPy wwrDVcC4cIFhYSfsO0BumEI65yu7a8aHbGfq2lW251UcoU48Z27ZUUZd2Dr6O/n8poQHbaTd 6bJJSjzGGHZVbRP9UQ3lkmkmc0+XCHmj5WhwNNYjgbbmML7y0fsJT5RgvefAIFfHBg7fTY/i kBEimoUsTEQz+N4hbKwo1hULfVxDJStE4sbPhjbsPCrlXf6W9CxSyQ0qmZ2bXsLQYRj2xqd1 bpA+1o1j2N4/au1R/uSiUFjewJdT/LX1EklKDcQwpk06Af/N7VZtSfEJeRV04unbsKVXWZAk uAJyDDKN99ziC0Wz5kcPyVD1HNf8bgaqGDzrv3TfYjwqayRFcMf7xJaL9xXedMcAEQEAAcLB XwQYAQgACQUCUuE2fwIbDAAKCRBlw/kGpdefoG4XEACD1Qf/er8EA7g23HMxYWd3FXHThrVQ HgiGdk5Yh632vjOm9L4sd/GCEACVQKjsu98e8o3ysitFlznEns5EAAXEbITrgKWXDDUWGYxd pnjj2u+GkVdsOAGk0kxczX6s+VRBhpbBI2PWnOsRJgU2n10PZ3mZD4Xu9kU2IXYmuW+e5KCA vTArRUdCrAtIa1k01sPipPPw6dfxx2e5asy21YOytzxuWFfJTGnVxZZSCyLUO83sh6OZhJkk b9rxL9wPmpN/t2IPaEKoAc0FTQZS36wAMOXkBh24PQ9gaLJvfPKpNzGD8XWR5HHF0NLIJhgg 4ZlEXQ2fVp3XrtocHqhu4UZR4koCijgB8sB7Tb0GCpwK+C4UePdFLfhKyRdSXuvY3AHJd4CP 4JzW0Bzq/WXY3XMOzUTYApGQpnUpdOmuQSfpV9MQO+/jo7r6yPbxT7CwRS5dcQPzUiuHLK9i nvjREdh84qycnx0/6dDroYhp0DFv4udxuAvt1h4wGwTPRQZerSm4xaYegEFusyhbZrI0U9tJ B8WrhBLXDiYlyJT6zOV2yZFuW47VrLsjYnHwn27hmxTC/7tvG3euCklmkn9Sl9IAKFu29RSo d5bD8kMSCYsTqtTfT6W4A3qHGvIDta3ptLYpIAOD2sY3GYq2nf3Bbzx81wZK14JdDDHUX2Rs 6+ahAA== In-Reply-To: <20250827123309.39699-1-frediano.ziglio@cloud.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Subject wants to be at least tools/libxc, and probably "Use superpages where possible on migrate/resume" On 27/08/2025 1:33 pm, Frediano Ziglio wrote: > Try to allocate larger order pages. > With some test memory program stressing TLB (many small random > memory accesses) you can get 15% performance improves. > On the first memory iteration the sender is currently sending > memory in 4mb aligned chunks which allows the receiver to > allocate most pages as 2mb superpages instead of single 4kb pages. It's critical to say somewhere that this is applicable to HVM guests. You've eluded to it, but it's important to state clearly that, for HVM guests, PAGE_DATA records contain metadata about GFNs in aligned 4M blocks.  This is why we don't even need to buffer a second record. It's also worth stating that 1G superpages are left for later. CC-ing Oleksii as release manager.  This is a fix for a (mis)feature which has been known for a decade (since Xen 4.6), and for which two series have been posted but not managed to get in.  Unlike those series, this is a very surgical fix that gets the majority of the perf win back, without the complexity of trying to guess at 1G pages. Therefore I'd like to request that it be considered for 4.21 at this juncture. > > Signed-off-by: Frediano Ziglio > --- > tools/libs/guest/xg_sr_restore.c | 39 ++++++++++++++++++++++++++++---- > 1 file changed, 35 insertions(+), 4 deletions(-) > > diff --git a/tools/libs/guest/xg_sr_restore.c b/tools/libs/guest/xg_sr_restore.c > index 06231ca826..8dcb1b19c5 100644 > --- a/tools/libs/guest/xg_sr_restore.c > +++ b/tools/libs/guest/xg_sr_restore.c > @@ -129,6 +129,8 @@ static int pfn_set_populated(struct xc_sr_context *ctx, xen_pfn_t pfn) > return 0; > } > > +#define IS_POWER_OF_2(n) (((n) & ((n) - 1)) == 0) > + > /* > * Given a set of pfns, obtain memory from Xen to fill the physmap for the > * unpopulated subset. If types is NULL, no page type checking is performed > @@ -141,6 +143,7 @@ int populate_pfns(struct xc_sr_context *ctx, unsigned int count, > xen_pfn_t *mfns = malloc(count * sizeof(*mfns)), > *pfns = malloc(count * sizeof(*pfns)); > unsigned int i, nr_pfns = 0; > + bool contiguous = true; > int rc = -1; > > if ( !mfns || !pfns ) > @@ -159,18 +162,46 @@ int populate_pfns(struct xc_sr_context *ctx, unsigned int count, > if ( rc ) > goto err; > pfns[nr_pfns] = mfns[nr_pfns] = original_pfns[i]; > + if ( pfns[nr_pfns] != pfns[0] + nr_pfns ) > + contiguous = false; > ++nr_pfns; > } > } > > if ( nr_pfns ) > { > - rc = xc_domain_populate_physmap_exact( > - xch, ctx->domid, nr_pfns, 0, 0, mfns); > + /* try optimizing using larger order */ > + rc = -1; > + /* > + * The "nr_pfns <= (1 << 18)" check is mainly for paranoia, it should > + * never happen, the sender would have to send a really large packet. > + */ > + if ( contiguous && nr_pfns <= (1 << 18) && This is an arbitrary limit, and to contradict the prior feedback given in private, the domain's MAX_ORDER isn't relevant here.  It's the toolstack's choice how to lay out the guest in memory. > + IS_POWER_OF_2(nr_pfns) && (pfns[0] & (nr_pfns - 1)) == 0 ) > + { > + const unsigned int extent_order = __builtin_ffs(nr_pfns) - 1; This (non-)loop isn't great.  You'll e.g. use 4k pages for the second 2M page of an HVM guest simply because the VGA hole exists in the first. I think you probably want something like: int populate_physmap_4k(ctx, nr, mfns); int populate_physmap_2M(ctx, nr, mfns); as simple wrappers around the raw hypercalls including transforming back the mfns[] array, and: int populate_phymap(...); with logic of the form:     while ( nr )     {         if ( nr < 512 ) /* Can't be a superpage */         {             populate_physmap_4k(ctx, i, mfns);             mfns += i;             nr -= i;             continue;         }         if ( !ALIGNED_2M(mfn) ) /* Populate up until a point that could be a superpage */         {             while ( !ALIGNED_2M(mfn + i) )                 i++;             populate_physmap_4k(ctx, i, mfns);             mfns += i;             nr -= i;         }         if ( nr >= 512 )         {             for ( i = 1; i < 512; ++i )                 if ( mfns[i] != mfns[0] + i )                     break;             if ( i == 512 )                 populate_physmap_2M(ctx, i, mfns); else populate_physmap_4k(...);             mfns += i;             nr -= i;         }     } Obviously with error handling, and whatever boundary conditions I've got wrong. 2M is the only size that matters (ignoring 1G which we've excluded for now), and this form will reconstruct more superpages for the guest than trying to do 4M allocations will. ~Andrew