[HN Gopher] Free interactive tool that shows you how PCIe lanes ...
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Free interactive tool that shows you how PCIe lanes work on
motherboards
Author : tagyro
Score : 261 points
Date : 2025-11-19 07:13 UTC (2 days ago)
(HTM) web link (mobomaps.com)
(TXT) w3m dump (mobomaps.com)
| sidewndr46 wrote:
| Wow, this is great! I don't know how they generate this but it's
| really impressive. One of the things that I've been surprised
| with is some older dual socket workstations have tons of PCI-E
| lanes, but none are hooked to the second CPU it seems
| nirav72 wrote:
| Nice! One suggestion - please add AM4 socket boards. With current
| memory prices, AM5 with DDR5 is becoming unattainable for some.
| DDR4 prices are rising as well. But not nearly as bad as DDR5.
| Dylan16807 wrote:
| So you're specifically considering the people that would have
| gone AM5 but are now looking at AM4 at the end of 2025 and into
| 2026?
|
| Is that a significant number of people? I kind of expect almost
| everyone that waited this long to sit tight on their current
| builds and keep waiting until RAM goes back down.
| dontlaugh wrote:
| Some don't already have a build.
|
| AM4 has high end gaming CPUs available and there are not yet
| major limitations besides future upgrades.
|
| AM4 parts probably makes the best value gaming build you can
| get right now.
| Dylan16807 wrote:
| > Some don't already have a build.
|
| The number of people that don't have a previous computer
| and are shopping based on individual parts is even smaller.
|
| > AM4 parts probably makes the best value gaming build you
| can get right now.
|
| Has the value improved in the last year and a half? The
| people that would want AM4 have had a long time to buy AM4.
| dontlaugh wrote:
| Yes, due to DDR5 prices going up after a period of people
| upgrading to AM5. There is currently a lot of second hand
| AM4 DDR4 stock, making the value particularly good.
|
| FWIW I recently upgraded and ended up getting something
| else, but mostly due to lack of availability of ITX AM4
| motherboards. I got one with a soldered high end laptop
| CPU, it was much cheaper.
| Dylan16807 wrote:
| DDR5 price increases don't make AM4 improve, they just
| make AM5 worse.
|
| Consistent drops from secondhand stock sound good.
| dontlaugh wrote:
| It makes AM5 less attractive and AM4 better value
| relatively. This situation will end eventually, of
| course.
| gr4vityWall wrote:
| > Has the value improved in the last year and a half?
|
| I'd say yes, my reasoning being:
|
| 1. DDR5 got more expensive;
|
| 2. 16 cores AM4 CPUs are cheaper, partially due to the
| release of the 5900XT (16C/32T CPU, basically a rebranded
| 5950X with 100mhz lower single-core boost clock);
|
| 3. Lots of gamers selling their used AM4 kits for good
| prices as they migrate to the 9800X3D.
| KronisLV wrote:
| If I needed a new build right now, it would most likely be
| AM4. Going for the latest gen would just be out of my price
| point, whereas AM4 still has plenty of good chips and
| motherboards out there.
| NedCode wrote:
| Legendary!
| asciii wrote:
| Warning: addicting site :)
| rao-v wrote:
| I've been struggling to find an AM5 board that can run three
| MI50s at 4x. This is perfect thank you.
|
| Him are you sure about some of the PCI slots? I think some marked
| as 4x get downgraded to 1x on these boards...
|
| Further edit - this maybe accurate - how are you getting this /
| confirming it?
| chainingsolid wrote:
| I would normally figure this out by reading motherboard
| manuals. Which for SKUs you can buy standalone tend to be on
| the manufacter's site with no account/paywall. They tend to
| include all the "if you populate this slot you lose xyz"
| language. Along with how to change PCIe lane bifercation in
| bios if nessesary.
| smcleod wrote:
| That is so incredibly useful, hardware vendors do such a bad job
| of properly advertising how many GPUs will actually work and with
| what combination of m.2 slots in use.
| consp wrote:
| Bifurcation support is also almost never mentioned, even if the
| bios supports it.
| SketchySeaBeast wrote:
| Yup. I've been lost for a while on how to properly set up my
| MSI X870 TOMAHAWK mobo, this makes it all clear. Boy is it a
| mess with all the bifurcation.
| mifreewil wrote:
| Very nice! Just a note (as the site says on bottom left side),
| this can vary depending on the CPU you use, would be nice to be
| able to select all different variations of supported CPUs as a
| future feature.
| tagyro wrote:
| For disclosure, this was created by "Ronin Wilde" -
| https://www.youtube.com/watch?v=cgdXj75VSMo
|
| I found it useful and thought others might also like it.
| throw7 wrote:
| I wish all manufacturers clearly gave info like this up front.
| AM4 boards would be nice.
| PunchyHamster wrote:
| Yeah my ASRock have nice map of the every lane and interface
| and where they are connected on the board. Especially important
| as some devices go thru second io expander
| matja wrote:
| How can I contribute the data for the boards I own which are not
| on the site?
| gitpusher wrote:
| Whoa. This is so cool and helpful. Too bad my board is Intel. Is
| there a way to contribute to this?
| tagyro wrote:
| I dropped a message to the creator :fingers_crossed: they open
| the motherboard database so we can make contributions
| rkagerer wrote:
| Can anyone recommend a specific, well-made, high-performance
| motherboard with loads of PCIe lanes and expansion slots, and
| sensible lane topology?
|
| All the motherboards these days make me feel claustrophobic. My
| current workstation is pretty old, but feels like it had more
| expansion capability (relative to its time) than what's on the
| market today.
| rkagerer wrote:
| Some builds I kept tabs on:
|
| Let's Encrypt documented their early 2021 whitebox that used
| 128 PCIe 4.0 lanes, mainly for storage:
| https://letsencrypt.org/2021/01/21/next-gen-database-servers...
|
| Troy Hunt (HaveIBeenPwned) recently solicited upgrade advice
| from the internet and settled on an Asus Pro WS TRX50-SAGE WIFI
| (which doesn't appear to be in the MoboMaps database yet):
| https://gist.github.com/troyhunt/a6e565981e4769976e9cffb705f...
| Aurornis wrote:
| You'll have to be more specific about your price range. There
| are a lot of server and workstation chipsets/platforms that
| will have a large number of PCIe lanes, but you will pay for
| them.
|
| I really suggest not seeking a lot of PCIe lanes unless you
| really need them right now, though. The price premium for a
| platform with a lot of extra PCIe is very steep once you get
| past consumer boards. It would be a shame to spend a huge
| premium on a server board and settle for slower older tech CPUs
| only to have all of those slots sit empty.
|
| It's a good idea to add up the PCIe devices you will use and
| the actual bandwidth they need. You lose very little by running
| a GPU in a PCIe x8 slot instead of a full x16 slot, for
| example. A 10G Ethernet card only needs 1 lane of PCIe 4.0.
| Even fast SSDs can get away with half of their lanes and you'll
| never notice except in rare cases of sustained large file
| transfers.
| shadowpho wrote:
| >Even fast SSDs can get away with half of their lanes and
| you'll never notice except in rare cases of sustained large
| file transfers
|
| Sorta yes but kinda the other way around -- you'll mostly
| notice in short high burst of I/O. This is mostly the case
| for people who use them to run remote mounted VM.
|
| Nowadays all nvme have a cache on board (ddr3 memory is
| common), which is how they manage to keep up with high speed.
| However once you exhaust the cache speeds drop dramatically.
|
| But your point is valid that very few people actually notice
| a difference
| wtallis wrote:
| You're pretty far off the mark about SSD caching. A
| majority of consumer SSDs are now DRAMless, and still can
| exceed PCIe 4.0 x4 bandwidth for sequential transfers. Only
| a seriously outdated SSD would still be using DDR3; good
| ones should be using LPDDR4 or maybe DDR4. And when a SSD
| _does_ have DRAM, it isn 't there for the sake of caching
| your data, it's for caching the driver's internal metadata
| that tracks the mapping of logical block addresses to
| physical NAND flash pages.
| shadowpho wrote:
| Here's a page comparison 8 modern SSD cache, notice how
| they all fall off once the cache is full.
|
| https://pcpartpicker.com/forums/topic/423337-animated-
| graphs...
| wtallis wrote:
| That has nothing to do with DRAM; that would be
| completely obvious if you stopped to think about the
| cache sizes implied by writing at 5-6GB/s for tens of
| seconds before speeds drop. Nobody's putting 100+ GB of
| DRAM on a single SSD. You get at most 1GB of DRAM per 1TB
| of NAND.
|
| What those graphs illustrate is _SLC caching_ : writing
| faster by storing one bit per NAND flash memory cell
| (imprecisely), then eventually re-packing that data to
| store three or four memory bits per cell (as is necessary
| to achieve the drive's nominal capacity). Note that this
| only directly affects _write_ operations; _reading_ data
| at several GB /s is possible even for data that's stored
| in TLC/QLC cells, and can be sustained for the entire
| capacity of the drive.
| rkagerer wrote:
| Sky's the limit. (Short of hiring a team of engineers to
| design and fab a one-off board, anyway).
|
| I appreciate your advice. I use the machine for a variety of
| different tasks, and am looking to accommodate at least two
| high-end GPU's (1 for passthrough to virtual machines for
| running things like Solidworks), a number of SSD's, and as
| many PCIe expansion cards as possible. Many of the cards are
| older-gen, so could be consolidated to just a few modern
| lanes if I could find an external expander with sufficiently
| generous capacity. Here's a quick inventory of what's in the
| existing box:
|
| - Mellanox Infiniband. For high-speed, low-latency
| networking... these days, probably replaceable with
| integrated NIC's, particularly if they come with RDMA.
|
| - High-performance RAID. I've found dedicated cards offer
| better features, performance, capacity, resilience and
| reliability than any of the mobo-integrated garbage I've
| tried over the years. Things like BBU's/SuperCaps, seamless
| migration and capacity upgrades, out-of-band monitoring, etc.
| e.g. I've taken my existing mass storage array created on a
| modest ARC-1231ML 15+ years ago, through several newer
| generations to an ARC-1883, with many disk and capacity
| upgrades along the way, but it's _still the same array_
| without ever having had to reformat and restore from scratch.
| Incidentally I 've been particularly happy with Areca's
| hardware, and they've even implemented some features I
| requested over the years (like the ability to hot-clone a
| replacement disk for one expected to fail soon then swap in
| the new one, without having to degrade the array and wait for
| a lengthy rebuild process that reduces your fault tolerance
| while hammering all member disks; as well as some other
| tweaks for better compatibility with tools like Hard Disk
| Sentinel). I notice they're finally starting to come out with
| controllers oriented to SSD's, like a PCI 5.0 product
| (https://www.areca.com.tw/products/nvme-1689-8N.html) for up
| to 8 x4 M.2 SSD's that boasts up to 60 GB/s, which is
| interesting (though the high-queue-depth random performance
| still doesn't match directly-plugged drives). I know
| software-RAID for the solid state stuff is also an option (as
| is just living without redundancy), but it's been convenient
| outsourcing the complexity.
|
| - Slim, low-performance accessory GPU for more displays
|
| - A few others this crowd would just laugh at me for (e.g. a
| PCI I/O card that includes a true parallel port, because
| nothing is more fun(tm) for hobbyist stuff and USB-based
| alternatives were found to have too much abstraction or
| latency; a SCSI adapter for an archaic piece of vintage
| hardware I'd love to keep installed permanently but there
| ain't space, and occasional one-off use stuff like a high-
| bandwidth digitizer).
|
| The motherboard had 6 PCIe slots, and I've got two more
| provided by an external PCIe expander (after accounting for
| the one lost for it's own connection). If I could find some
| kind of expander that took a single PCIe 5.0 slot and turned
| it into half a dozen PCIe 3.0 slots (some full-width) I'd be
| set.
|
| I know I'm at the crazy end of how-much-crap-can-you-jam-in-
| one-PC, but it still seems bizarre to me that newer boards
| have so many fewer slots yet feel lane-constrained, when
| between leading-edge SSD's and high-bandwidth GPU's the
| demand for more lanes is skyrocketing. When I built the
| previous PC it felt tight but doable... these days it feels
| like I can barely accommodate the level of graphics and
| storage I'd like, and by the time I do, there's nothing left
| for anything else. Granted it's been a few years since I got
| my hands dirty with this stuff, so maybe I'm just doing it
| wrong?
|
| And yes, I've heard of USB... and have a bazillion devices
| plugged in (including some of exotic ones like an LCD
| display, logic analyzer, and a _legit_ floppy drive that does
| get used once in a blue moon like when I need to make a
| memtest86 boot disk for a vintage PC). I 've actually found
| some motherboards have issues where the USB stack gets flakey
| once you have too many devices connected (even using powered
| hubs to mitigate power constraints).
|
| Ok... go ahead and have at me; tell me I'm old and dusty and
| I should take my one GPU and one SSD and be happy with them
| ;-).
| zamadatix wrote:
| I just built a 4th gen Epyc build with 22x NVMe drives and
| a dual port 40G NIC. It was a FAR superior experience to
| trying to use prosumer parts, PCIe splitters, etc and
| didn't end up costing as much as I thought it would (though
| the DDR5 RAM has went up 2x in the last month since I
| bought haha).
|
| Side note on RAID: Motherboard integrated garbage is still
| meh, but hardware options are also pretty meh. I just use
| software options like ZFS on Linux (or mdraid if I just
| need a fat RAID 0 with no protection) and get fantastic
| speeds, portability, no artificial drive topology
| restrictions, and no additional latency to accessing the
| drives over PCIe. On Windows the equivalents would be ReFS
| Storage Pools or whatever the software RAID 0 in the disk
| manager was called.
|
| If you're looking for cheaper and "not the latest gen" the
| search tugm4770 on Ebay and filter to the CPU you want.
| They have MB bundles for standard EATX chassis or bundles
| with used supermicro servers+PSUs as well. I went the EATX
| route and put it in a standard PC case, it's crazy how
| quiet it runs (it's in my living room even)! I used to buy
| from this seller for the lab at my previous job, but
| decided to go all in at home this time. Never had trouble
| with them and it is by far the cheapest way to get these
| kinds of setups (just avoid the really old Epyc
| generations).
|
| If the sky is truly the limit and you want the absolute
| best of the best, get a current gen high frequency
| optimized Epyc new (don't know the easiest place to do
| that). You can also do the Threadripper PRO, just depends.
|
| Both of these Epyc options have a "how sky high?" option of
| getting a dual socket MB, which can give you up to 192 PCIe
| lanes instead of up to 128. You have to populate both
| sockets, and the physical footprint starts to become
| monstrous (especially if you use all the lanes).
| crote wrote:
| For a lot of enthousiasts the problem genuinely is in the
| lane count.
|
| For example, putting a cheap 2nd-hand dual 25G NIC in a DIY
| server is quite attractive. But those are using PCIe Gen3 -
| so unless you're giving it 8 lanes it is being bottlenecked.
|
| Same on the storage side: that PCIe Gen4 x4 slot might
| technically have enough bandwidth for four SSDs in most
| storage applications, but the board doesn't support
| bifurcating it.
|
| The total platform bandwidth is plenty, it just isn't
| available in the way I want to use it.
| michaelt wrote:
| Given that you've said 'workstation', if you've got a spare
| $5000, a Threadripper Pro comes with 128 PCIe 5.0 lanes.
|
| This means you can get a motherboard like the "Asus Pro WS
| WRX90E-SAGE SE" which dedicates 104 lanes to seven PCIe slots
| and 16 lanes to four M.2 slots.
|
| For more like $3000 you can get a non-Pro Threadripper; the
| "Asus Pro WS TRX50-SAGE" has a more restrained 48 PCIe 5.0 and
| 32 PCIe 4.0 lanes, meaning the board's five PCIe slots and
| three M.2 slots have a mixture of speeds and lanes.
|
| The rest of the market seems to think you just want to plug in
| one huge four-slot GPU and perhaps one other card.
| rkagerer wrote:
| Now we're talking - thanks!
|
| (ps. I don't suppose they make a "supersized" version of that
| board with a gap beside the first one or two GPU slots? So
| you can install a couple double-width cards without losing
| the underlying slots? Or a good source for a single-width,
| high-end GPU like the Inno3D RTX 5090 iChill Frostbite Pro?)
| PeterStuer wrote:
| You would typically use good riser cables for that. Spacing
| out the MB pcb itself is not an effective option.
| michaelt wrote:
| The card in the bottom slot can overhang the board, if you
| only need to add a single multi-slot card.
|
| Beyond that point you're probably looking at getting a
| server. Boards like the X9DRG-OF have eight dual-width
| slots, and you can get them in a quadruple-power-supply
| configuration - which you'll want when you add up the power
| consumption of a bunch of GPUs. Anything rack-mount will be
| very noisy though.
|
| Another options is a cryptocurrency mining style 'open
| frame' system with PCIe risers. Google that and you'll find
| some crazy setups enthusiasts have posted to reddit.
|
| Or you can just run something in the cloud - you can buy a
| lot of cloud time for $6000.
| hengheng wrote:
| Look into CXL, Oculink, and riser cables.
| PeterStuer wrote:
| If you _really_ need lots of pcie lanes, you are going to be
| moving up to the TRX50 (or used TRX40) and its ilk. Different
| price ranges from your typical enthusiast MB though.
| notpublic wrote:
| Check out the following for AM5 motherboards. It helped me a
| lot when I was in the market. Seems to be well maintained
| still:
|
| https://docs.google.com/spreadsheets/d/1NQHkDEcgDPm34Mns3C93...
|
| I ended up getting ASRock X870E Taichi Lite. The main reason to
| get it was because it had 2 CPU x8 slots which are spaced
| perfectly for an Nvidia NVLink. And, they are Gen5 PCIe.
| temp0826 wrote:
| Probably a good thing SLI fell out of fashion. No consumer boards
| with multiple 16x, but a few with 2 8x (gated behind a "mode"
| switch). A few years ago it was looking like we were on our way
| to full 4 16x slots. For cuda/llm/whatever does it really matter
| if the cards are in 1x slots?
| hoss1474489 wrote:
| GPUs in 16x slots is still important for LLM stuff, especially
| multi-GPU, where lots of data needs to move between cards
| during computation.
| Dylan16807 wrote:
| Depends on what you're doing. I'm pretty sure the bandwidth
| for inference isn't much.
| eurekin wrote:
| Depends, if it's tensor parallel or pipeline parallel. Only
| PP doesn't pass too much. TP does
| __alexs wrote:
| A 16x PCIE 6.0 setup has more bandwidth than any dual channel
| DDR5 memory kit.
| tryauuum wrote:
| ... shouldn't the logic be opposite? "Bad that SLI went out of
| fashion, there's no way for two GPUs to communicate fast over
| pcie, and SLI would allow such fast bridge"
| wtallis wrote:
| Whether or not SLI remained viable for gaming, Broadcom was
| going to jack up the prices on PCIe switches to the
| enterprise-only range. _That 's_ the real reason why consumer
| motherboards don't have more GPU slots. Mainstream consumer
| CPU sockets never had a wealth of PCIe lanes, there was just
| a brief span of years where PCIe switches were cheap so high-
| end consumer boards could offer several x8 or x16 slots
| (sharing bandwidth in ways that make diagrams like these
| important).
|
| In previous decades, non-mainstream CPU sockets were also
| more accessible to consumer budgets; first-gen Threadripper
| started at only 8 cores, so it was possible to pay extra for
| more memory channels and IO lanes without also buying an
| excess of CPU cores. But that had little to do with the
| popularity or viability of multi-GPU consumer systems.
| crote wrote:
| But PCIe switches are now more common than ever. How else
| do you think those high-end consumer boards are able to
| provide six M.2 slots?
| bayindirh wrote:
| Not all PCIe switches are made equal. IIRC, there are two
| kinds, and only the expensive one can work without adding
| excessive latency, important for GPUs.
|
| NVMe traffic is very tame when compared to GPU traffic.
| wtallis wrote:
| PCIe switches with _current-generation_ link speeds and
| high lane counts are prohibitively expensive and have
| been absent from consumer motherboards since PCIe gen3
| showed up.
|
| The chipsets on consumer motherboards are pretty much
| PCIe switches plus some SATA and USB controllers, but
| they're clearly in a different league from anything
| that's relevant to connecting GPUs. The host interfaces
| are x4 or occasionally x8, and the downstream links don't
| support links wider than x4, and at most a few of those.
| The link speeds are often a generation (sometimes _two_ )
| behind what the CPU's PCIe lanes support. The high-end
| motherboards for AMD's consumer platform support more SSD
| slots by _daisy-chaining_ a second chipset off the first;
| you get more M.2 slots but it 's all still sharing a
| single PCIe gen4 x4 link to the CPU.
|
| In the PCIe gen2 era, it _was_ common to see high-end
| consumer motherboards include a 48-lane PCIe switch to
| take x16 from the processor and fan it out to two x16
| slots or some combination of x16 and x8 slots. That kind
| of connectivity _has_ vanished from consumer
| motherboards, and isn 't really common in the server or
| workstation markets, either. 48-lane and larger PCIe
| switches exist, but are mostly just used for servers to
| connect lots of NVMe SSDs.
| cjensen wrote:
| It's the other way around. SLI falling out of fashion is why
| there are no consumer boards with multiple x16 slots. There's
| no longer any demand for it on the consumer side, so the CPU
| vendors only provide lots of PCIe lanes for expensive chips.
|
| On the server side, seven x16 slot motherboards exist.
| Dylan16807 wrote:
| I would expect x8 at 5.0 speeds to be plenty for SLI. That's
| twice as fast as x16 slots were around the end of the SLI era.
| tripdout wrote:
| Very cool. Seeing how almost everything from WiFi, to NVME SSDs,
| (to apparently USB ports sometimes?) are connected to it, is PCIe
| the only high-speed interconnect we have for peripherals to
| communicate with modern CPUs?
| baby_souffle wrote:
| > PCIe the only high-speed interconnect we have for peripherals
| to communicate with modern CPUs?
|
| In a pedantic/technical sense, no. Practically speaking though,
| yes.
| stinkbeetle wrote:
| The high speed signals that come out of mainstream CPU chips
| are generally DDR, SMP, and PCIe. Outside of a very few exotic
| things that use QPI or HT to connect, or exotic storage might
| use DDR, yes high speed off-chip peripherals use PCIe.
|
| NVLink is another one you might have heard of, although it
| might also fall in the exotic category. I think some systems
| take AXI off-chip too. So there's various other weird and
| wonderful things. But none you're likely to have in your PC I
| think.
|
| On-chip is another story, you can connect USB or NVMe or GPU
| "peripherals" using an on-chip interconnect type. But I guess
| you are asking about off-chip.
| crote wrote:
| USB4 needs PCIe because its Thunderbolt part has PCIe
| tunneling.
| max002 wrote:
| So cool xD i think you could take it to tool with premoum
| features for people who are learning :)
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