Question about cacheline size in PCIe SAS card
Hi all, we found a question about PCIe cacheline, the cacheline here is mean the configure space register at offset 0x0C in type 0 and type 1 configure space header.
We did a hotplug in our platform for PCIe SAS controller, this sas controller has SSD disks and the disk sector is 520 bytes. Defaultly, BIOS set cacheline size to 64bytes, we test the IO read(io size is 128k/256k), the bandwith is 6G. After hotplug, the cacheline size in SAS controller changes to 0(default after #RST), and we test the IO read again, the bandwith changes to 5.2G.
We Tested other SAS controller which is not 520 bytes sector, we didn't found this issue, and I grep the PCI_CACHE_LINE_SIZE in kernel, I found most of code change the PCI_CACHE_LINE_SIZE are device driver, like net, ata, and some arm pci controller.
In PCI 3.0 spec, I found there are descriptions about cacheline size releated to performance, but in PCIe 3.0 spec, there is nothing related to cacheline size.
I wonder what's cacheline register roles in PCIe spec, how can we use it correctly ?
Thanks!
Yijing.
Quoted 9 lines · wangyijing
Hi all, we found a question about PCIe cacheline, the cacheline here is mean the configure space register at offset
0x0Cin type 0 and type 1 configure space header.We did a hotplug in our platform for PCIe SAS controller, this sas controller has SSD disks and the disk sector is 520 bytes. Defaultly, BIOS set cacheline size to 64bytes, we test the IO read(io size is 128k/256k), the bandwith is 6G. After hotplug, the cacheline size in SAS controller changes to 0(default after #RST), and we test the IO read again, the bandwith changes to 5.2G.
Quoted 6 lines
We Tested other SAS controller which is not 520 bytes sector, we didn't found this issue, and I grep the
PCI_CACHE_LINE_SIZEin kernel, I found most of code change thePCI_CACHE_LINE_SIZEare device driver, like net, ata, and some arm pci controller.In PCI 3.0 spec, I found there are descriptions about cacheline size releated to performance, but in PCIe 3.0 spec, there is nothing related to cacheline size.
Not quite true: sec 7.5.1.3 of PCIe r3.0 says:
This field [Cache Line Size] is implemented by PCI Express devices as a read-write field for legacy compatibility purposes but has no effect on any PCI Express device behavior.
Unless your SAS controller is doing something wrong, I suspect something other than Cache Line Size is responsible for the difference in performance.
After hot-add of your controller, Cache Line Size is probably zero because Linux doesn't set it. What happens if you set it manually using "setpci"? Does that affect the performance?
You might look at the MPS and MRRS settings in the two scenarios also.
You could try collecting the output of "lspci -vvxxx" for the whole system in the default case and again after the hotplug, and then compare the two for differences.
Bjorn
Hi Bjorn, thanks for your comment!
Quoted 14 lines · Bjorn Helgaas
Quoted 13 lines · wangyijing
Hi all, we found a question about PCIe cacheline, the cacheline here is mean the configure space register at offset
0x0Cin type 0 and type 1 configure space header.We did a hotplug in our platform for PCIe SAS controller, this sas controller has SSD disks and the disk sector is 520 bytes. Defaultly, BIOS set cacheline size to 64bytes, we test the IO read(io size is 128k/256k), the bandwith is 6G. After hotplug, the cacheline size in SAS controller changes to 0(default after #RST), and we test the IO read again, the bandwith changes to 5.2G.
We Tested other SAS controller which is not 520 bytes sector, we didn't found this issue, and I grep the
PCI_CACHE_LINE_SIZEin kernel, I found most of code change thePCI_CACHE_LINE_SIZEare device driver, like net, ata, and some arm pci controller.
Quoted 8 lines
Quoted 2 lines
In PCI 3.0 spec, I found there are descriptions about cacheline size releated to performance, but in PCIe 3.0 spec, there is nothing related to cacheline size.
Not quite true: sec 7.5.1.3 of PCIe r3.0 says:
This field [Cache Line Size] is implemented by PCI Express devices as a read-write field for legacy compatibility purposes but has no effect on any PCI Express device behavior.
Oh, sorry, I searched the key word "cacheline" in PCIe spec, according to this description, there is no effect on any PCIe device.
Quoted 8 lines
Unless your SAS controller is doing something wrong, I suspect something other than Cache Line Size is responsible for the difference in performance.
After hot-add of your controller, Cache Line Size is probably zero because Linux doesn't set it. What happens if you set it manually using "setpci"? Does that affect the performance?
Yes, after hotplug, the cacheline size is reset to 0, linux doesn't touch it, and we tried to change cacheline size to 64 bytes by setpci, if we test the IO at this time, the IO bandwith is still 5.2G, but if we reset the firmware after change the cacheline size to 64 bytes, then test IO bandwith again, the IO bandwith would reach the 6G again.
Quoted 2 lines
You might look at the MPS and MRRS settings in the two scenarios also.
There is no difference for MPS and MRRS in the two scenarios, hotplug driver restore them according their original values.
Quoted 4 lines
You could try collecting the output of "lspci -vvxxx" for the whole system in the default case and again after the hotplug, and then compare the two for differences.
Yes, I did, and there is no other significant difference found except cacheline size.
I suspect the SAS controller internal issues hurt the performance.
The normal config space after system boot up
13:00.0 Serial Attached SCSI controller: PMC-Sierra Inc. Device 8072 (rev 06)
Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx+
Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
Latency: 0, Cache Line Size: 64 bytes
Interrupt: pin A routed to IRQ 34
Region 0: Memory at 97000000 (64-bit, non-prefetchable) [size=64K]
Region 2: Memory at 97010000 (64-bit, non-prefetchable) [size=64K]
Expansion ROM at 97100000 [disabled] [size=1M]
Capabilities: [80] Power Management version 3
Flags: PMEClk- DSI- D1+ D2- AuxCurrent=0mA PME(D0+,D1+,D2-,D3hot+,D3cold-)
Status: D0 NoSoftRst+ PME-Enable- DSel=0 DScale=0 PME-
Capabilities: [88] Vital Product Data
Unknown small resource type 00, will not decode more.
Capabilities: [90] MSI: Enable- Count=1/32 Maskable+ 64bit+
Address: 0000000000000000 Data: 0000
Masking: 00000000 Pending: 00000000
Capabilities: [b0] MSI-X: Enable+ Count=64 Masked-
Vector table: BAR=0 offset=00000400
PBA: BAR=0 offset=00000800
Capabilities: [c0] Express (v2) Endpoint, MSI 00
DevCap: MaxPayload 512 bytes, PhantFunc 0, Latency L0s <4us, L1 <1us
ExtTag+ AttnBtn- AttnInd- PwrInd- RBE+ FLReset-
DevCtl: Report errors: Correctable- Non-Fatal- Fatal- Unsupported-
RlxdOrd+ ExtTag- PhantFunc- AuxPwr- NoSnoop+
MaxPayload 256 bytes, MaxReadReq 512 bytes
DevSta: CorrErr+ UncorrErr- FatalErr- UnsuppReq+ AuxPwr- TransPend-
LnkCap: Port #0, Speed 8GT/s, Width x8, ASPM unknown, Latency L0 <1us, L1 <1us
ClockPM- Surprise- LLActRep- BwNot-
LnkCtl: ASPM Disabled; RCB 64 bytes Disabled- Retrain- CommClk-
ExtSynch- ClockPM- AutWidDis- BWInt- AutBWInt-
LnkSta: Speed 8GT/s, Width x8, TrErr- Train- SlotClk- DLActive- BWMgmt- ABWMgmt-
DevCap2: Completion Timeout: Range B, TimeoutDis+
DevCtl2: Completion Timeout: 50us to 50ms, TimeoutDis-
LnkCtl2: Target Link Speed: 8GT/s, EnterCompliance- SpeedDis-, Selectable De-emphasis: -6dB
Transmit Margin: Normal Operating Range, EnterModifiedCompliance- ComplianceSOS- Compliance De-emphasis: -6dB
LnkSta2: Current De-emphasis Level: -6dB, EqualizationComplete+, EqualizationPhase1+
EqualizationPhase2+, EqualizationPhase3+, LinkEqualizationRequest-
Capabilities: [100 v2] Advanced Error Reporting
UESta: DLP- SDES- TLP- FCP- CmpltTO- CmpltAbrt- UnxCmplt- RxOF- MalfTLP- ECRC- UnsupReq- ACSViol-
UEMsk: DLP- SDES- TLP- FCP- CmpltTO- CmpltAbrt- UnxCmplt- RxOF- MalfTLP- ECRC- UnsupReq- ACSViol-
UESvrt: DLP+ SDES+ TLP+ FCP+ CmpltTO- CmpltAbrt- UnxCmplt- RxOF+ MalfTLP+ ECRC- UnsupReq- ACSViol-
CESta: RxErr- BadTLP- BadDLLP- Rollover- Timeout- NonFatalErr-
CEMsk: RxErr- BadTLP- BadDLLP- Rollover- Timeout- NonFatalErr+
AERCap: First Error Pointer: 00, GenCap+ CGenEn- ChkCap+ ChkEn-
Capabilities: [300 v1] #19
Kernel driver in use: quark
The config space after hotplug
13:00.0 Serial Attached SCSI controller: PMC-Sierra Inc. Device 8072 (rev 06)
Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx+
Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
Latency: 0
Interrupt: pin A routed to IRQ 34
Region 0: Memory at 97100000 (64-bit, non-prefetchable) [size=64K]
Region 2: Memory at 97110000 (64-bit, non-prefetchable) [size=64K]
Expansion ROM at 97000000 [size=1M]
Capabilities: [80] Power Management version 3
Flags: PMEClk- DSI- D1+ D2- AuxCurrent=0mA PME(D0+,D1+,D2-,D3hot+,D3cold-)
Status: D0 NoSoftRst+ PME-Enable- DSel=0 DScale=0 PME-
Capabilities: [88] Vital Product Data
Unknown small resource type 00, will not decode more.
Capabilities: [90] MSI: Enable- Count=1/32 Maskable+ 64bit+
Address: 0000000000000000 Data: 0000
Masking: 00000000 Pending: 00000000
Capabilities: [b0] MSI-X: Enable+ Count=64 Masked-
Vector table: BAR=0 offset=00000400
PBA: BAR=0 offset=00000800
Capabilities: [c0] Express (v2) Endpoint, MSI 00
DevCap: MaxPayload 512 bytes, PhantFunc 0, Latency L0s <4us, L1 <1us
ExtTag+ AttnBtn- AttnInd- PwrInd- RBE+ FLReset-
DevCtl: Report errors: Correctable- Non-Fatal- Fatal- Unsupported-
RlxdOrd+ ExtTag- PhantFunc- AuxPwr- NoSnoop+
MaxPayload 256 bytes, MaxReadReq 512 bytes
DevSta: CorrErr- UncorrErr- FatalErr- UnsuppReq- AuxPwr- TransPend-
LnkCap: Port #0, Speed 8GT/s, Width x8, ASPM unknown, Latency L0 <1us, L1 <1us
ClockPM- Surprise- LLActRep- BwNot-
LnkCtl: ASPM Disabled; RCB 64 bytes Disabled- Retrain- CommClk-
ExtSynch- ClockPM- AutWidDis- BWInt- AutBWInt-
LnkSta: Speed 8GT/s, Width x8, TrErr- Train- SlotClk- DLActive- BWMgmt- ABWMgmt-
DevCap2: Completion Timeout: Range B, TimeoutDis+
DevCtl2: Completion Timeout: 50us to 50ms, TimeoutDis-
LnkCtl2: Target Link Speed: 8GT/s, EnterCompliance- SpeedDis-, Selectable De-emphasis: -6dB
Transmit Margin: Normal Operating Range, EnterModifiedCompliance- ComplianceSOS- Compliance De-emphasis: -6dB
LnkSta2: Current De-emphasis Level: -6dB, EqualizationComplete+, EqualizationPhase1+
EqualizationPhase2+, EqualizationPhase3+, LinkEqualizationRequest-
Capabilities: [100 v2] Advanced Error Reporting
UESta: DLP- SDES- TLP- FCP- CmpltTO- CmpltAbrt- UnxCmplt- RxOF- MalfTLP- ECRC- UnsupReq- ACSViol-
UEMsk: DLP- SDES- TLP- FCP- CmpltTO- CmpltAbrt- UnxCmplt- RxOF- MalfTLP- ECRC- UnsupReq- ACSViol-
UESvrt: DLP+ SDES+ TLP+ FCP+ CmpltTO- CmpltAbrt- UnxCmplt- RxOF+ MalfTLP+ ECRC- UnsupReq- ACSViol-
CESta: RxErr- BadTLP- BadDLLP- Rollover- Timeout- NonFatalErr-
CEMsk: RxErr- BadTLP- BadDLLP- Rollover- Timeout- NonFatalErr+
AERCap: First Error Pointer: 00, GenCap+ CGenEn- ChkCap+ ChkEn-
Capabilities: [300 v1] #19
Kernel driver in use: quark
Thanks!
Yijing.
Quoted 5 lines
Bjorn
.
Quoted 38 lines · wangyijing
Hi Bjorn, thanks for your comment!
Quoted 14 lines · Bjorn Helgaas
Quoted 13 lines · wangyijing
Hi all, we found a question about PCIe cacheline, the cacheline here is mean the configure space register at offset
0x0Cin type 0 and type 1 configure space header.We did a hotplug in our platform for PCIe SAS controller, this sas controller has SSD disks and the disk sector is 520 bytes. Defaultly, BIOS set cacheline size to 64bytes, we test the IO read(io size is 128k/256k), the bandwith is 6G. After hotplug, the cacheline size in SAS controller changes to 0(default after #RST), and we test the IO read again, the bandwith changes to 5.2G.
We Tested other SAS controller which is not 520 bytes sector, we didn't found this issue, and I grep the
PCI_CACHE_LINE_SIZEin kernel, I found most of code change thePCI_CACHE_LINE_SIZEare device driver, like net, ata, and some arm pci controller.Quoted 8 lines
Quoted 2 lines
In PCI 3.0 spec, I found there are descriptions about cacheline size releated to performance, but in PCIe 3.0 spec, there is nothing related to cacheline size.
Not quite true: sec 7.5.1.3 of PCIe r3.0 says:
This field [Cache Line Size] is implemented by PCI Express devices as a read-write field for legacy compatibility purposes but has no effect on any PCI Express device behavior.
Oh, sorry, I searched the key word "cacheline" in PCIe spec, according to this description, there is no effect on any PCIe device.
Quoted 8 lines
Unless your SAS controller is doing something wrong, I suspect something other than Cache Line Size is responsible for the difference in performance.
After hot-add of your controller, Cache Line Size is probably zero because Linux doesn't set it. What happens if you set it manually using "setpci"? Does that affect the performance?
Quoted 5 lines
Yes, after hotplug, the cacheline size is reset to 0, linux doesn't touch it, and we tried to change cacheline size to 64 bytes by setpci, if we test the IO at this time, the IO bandwith is still 5.2G, but if we reset the firmware after change the cacheline size to 64 bytes, then test IO bandwith again, the IO bandwith would reach the 6G again.
OK, that sounds like the category of "your controller doing something wrong," namely, it is somehow dependent on the Cache Line Size when it shouldn't be.
If you change Linux to set the Cache Line Size during hot-add, does that fix it? I assume you might still need to reset the firmware to make the card notice the change?
I wonder how this works in the non-hotplug case. Does the BIOS reset the firmware somehow after setting Cache Line Size? Is there an option ROM that might do this?
Maybe the quark driver needs a quirk in its probe routine that sets the Cache Line Size and resets the firmware?
Bjorn
Hi Bjorn, we confirmed this issue, it's caused by SAS controller internal desgin, following is the SAS FAE mail,
=================================================================================
Hi Jianghong,
i had confirmed that BDMA module do use this register, it will have performance improve for 520B sector.
if the sector size is 512B, then you have no issue with TLP alignment (because it will be in multiple of the TLP sizes). Problem happens when your sector size is not multiple of the TLP sizes, which means TLP is not aligned, to had a good performance, The BDMA algorithm determines how the engine aligns the TLPs and whether or not the enhanced BDMA algorithm is being turned on, it depends on the cache line size, it use this value as TLP alignment, and then IO performance will be different.
=================================================================================
So it's not a Linux PCIe issue, thanks very much for your comments and analysis!
Thanks!
Yijing.
Quoted 64 lines · Bjorn Helgaas
Quoted 38 lines · wangyijing
Hi Bjorn, thanks for your comment!
Quoted 14 lines · Bjorn Helgaas
Quoted 13 lines · wangyijing
Hi all, we found a question about PCIe cacheline, the cacheline here is mean the configure space register at offset
0x0Cin type 0 and type 1 configure space header.We did a hotplug in our platform for PCIe SAS controller, this sas controller has SSD disks and the disk sector is 520 bytes. Defaultly, BIOS set cacheline size to 64bytes, we test the IO read(io size is 128k/256k), the bandwith is 6G. After hotplug, the cacheline size in SAS controller changes to 0(default after #RST), and we test the IO read again, the bandwith changes to 5.2G.
We Tested other SAS controller which is not 520 bytes sector, we didn't found this issue, and I grep the
PCI_CACHE_LINE_SIZEin kernel, I found most of code change thePCI_CACHE_LINE_SIZEare device driver, like net, ata, and some arm pci controller.Quoted 8 lines
Quoted 2 lines
In PCI 3.0 spec, I found there are descriptions about cacheline size releated to performance, but in PCIe 3.0 spec, there is nothing related to cacheline size.
Not quite true: sec 7.5.1.3 of PCIe r3.0 says:
This field [Cache Line Size] is implemented by PCI Express devices as a read-write field for legacy compatibility purposes but has no effect on any PCI Express device behavior.
Oh, sorry, I searched the key word "cacheline" in PCIe spec, according to this description, there is no effect on any PCIe device.
Quoted 8 lines
Unless your SAS controller is doing something wrong, I suspect something other than Cache Line Size is responsible for the difference in performance.
After hot-add of your controller, Cache Line Size is probably zero because Linux doesn't set it. What happens if you set it manually using "setpci"? Does that affect the performance?
Quoted 5 lines
Yes, after hotplug, the cacheline size is reset to 0, linux doesn't touch it, and we tried to change cacheline size to 64 bytes by setpci, if we test the IO at this time, the IO bandwith is still 5.2G, but if we reset the firmware after change the cacheline size to 64 bytes, then test IO bandwith again, the IO bandwith would reach the 6G again.
OK, that sounds like the category of "your controller doing something wrong," namely, it is somehow dependent on the Cache Line Size when it shouldn't be.
If you change Linux to set the Cache Line Size during hot-add, does that fix it? I assume you might still need to reset the firmware to make the card notice the change?
I wonder how this works in the non-hotplug case. Does the BIOS reset the firmware somehow after setting Cache Line Size? Is there an option ROM that might do this?
Maybe the quark driver needs a quirk in its probe routine that sets the Cache Line Size and resets the firmware?
Bjorn
.